I feel that the right number of decks for a person is contingent on a number of factors. This can be budget (though proxies can help with some staples shared between decks as some people mentioned above) or time to play (no sense in having 25 decks if you only play 2 or 3 games a week) or just wanting to have a greater focus on certain decks. Some people will find that playing and perfecting a smaller number of decks is the most satisfying while others like having a greater variety at the cost of proficiency.
I used to be in the camp of wanting a greater number of decks for variety's sake, but I have scaled back quite a bit. I no longer build decks just to play them for a couple times and then dismantle them. It does happen, but I often only have one "new" deck at a time while keeping a consistent rotation of other decks. I usually go for new decks that offer something a little more unique that my existing decks don't.
For example, I currently have 12 decks and am working on a Yasova Dragonclaw deck. I basically have it completed but I just need to order a couple cards. Luckily I already have all the expensive cards. The reason I went with Yasova (after looking at a couple different generals) is that she is low to the ground and plays around with more "Threaten" effects that I haven't really done with any existing decks.
I generally try to keep my decks to 12 or less. I have 9 decks that are either fully foiled or pretty close to it and these are meant to be the "permanent" decks in my rotation. The other 3 are ones that were built for something new or as a specific answer to other decks in my meta. These are the 3 "flex slots" as it were with my decks. These are Prime Speaker Zegana, Darien, King of Kjeldor, and Mina and Denn, Wildborn. Once I get Yasova finished, I will play her a little and then determine if she is a good fit to replace one of my other decks. If so, I will take apart the deck she replaces. If not, I will take her apart. I plan on taking apart Mina and Denn right now as it is the least flavorful or unique deck in all the ones I have built. Prime Speaker is a potential cut as well.
I like to keep a maximum of 12 decks for a couple reasons. One is that I have it set up to be able to carry 12 decks, dice, tokens, and 2 playmats in my backpack. It works pretty well. I have a little space to carry an extra deck or two for early testing, but 12 is the right number. I also don't like having too many more as then it takes longer to rotate through the decks and I end up with decks I never play. That is the problem I had when I had upwards of 17 decks. Within the 12 decks I have, 3 are my "go to" decks: Alesha, Who Smiles at Death, Sidisi, Brood Tyrant, and Ephara, God of the Polis. These are the ones that get played the most often and I usually start a night out with either Alesha or Sidisi.
One is my more casual Garza Zol, Plague Queen deck and is meant mainly for unknown metas. It is meant to be lower powered so I don't come out swinging too fast if the meta isn't used to it and I would rather lose with an under-powered deck the first time in a new meta than crush with an overpowered one. It does have some power behind it, but it is a fun Vampire tribal deck that allows me to gauge the meta. I can then change the decks I play and the way I play them to suit the meta a little better. It is not played often due to not often playing in new locations, but I like having it just in case and it is a fun deck to end a night on sometimes.
My Narset, Enlightened Master deck exists solely to fight against players who insist on infinite combos or ways to win before turn 4. It is not a play style I enjoy so I don't play it often. but I keep it around just in case.
Otherwise, the rest are Thassa, God of the Sea. Gisela, Blade of Goldnight, Karametra, God of Harvests, and The Gitrog Monster. These have a decent power level and they are fun to play. I generally play Karametra and Gisela the most of these as they are the most fin to play against. Thassa is Mono-U control and can be frustrating to play against (but it is a play style I enjoy) so she isn't played frequently and The Gitrog is probably my favorite deck after Sidisi. The main problem with Gitrog is that even without infinite combos (I intentionally omitted Dakmor Salvage for example) the turns can often take a while to complete. I used to play in a "league" where we had timed rounds so I didn't want to take a bunch of time taking long turns. Now that I play in a different store where the games go as long as they need to, I might play this deck more often.
So I guess this is a long way to say that, for me, anything more than 12 decks is too many. Since 2 decks (Narset and Garza Zol) exists for fairly specific reasons, I guess I could say that a rotation of 10 decks is the max I want to go. After years of playing and being above and below this number, this so far has felt like the right number.
I used to be in the camp of wanting a greater number of decks for variety's sake, but I have scaled back quite a bit. I no longer build decks just to play them for a couple times and then dismantle them. It does happen, but I often only have one "new" deck at a time while keeping a consistent rotation of other decks. I usually go for new decks that offer something a little more unique that my existing decks don't.
For example, I currently have 12 decks and am working on a Yasova Dragonclaw deck. I basically have it completed but I just need to order a couple cards. Luckily I already have all the expensive cards. The reason I went with Yasova (after looking at a couple different generals) is that she is low to the ground and plays around with more "Threaten" effects that I haven't really done with any existing decks.
I generally try to keep my decks to 12 or less. I have 9 decks that are either fully foiled or pretty close to it and these are meant to be the "permanent" decks in my rotation. The other 3 are ones that were built for something new or as a specific answer to other decks in my meta. These are the 3 "flex slots" as it were with my decks. These are Prime Speaker Zegana, Darien, King of Kjeldor, and Mina and Denn, Wildborn. Once I get Yasova finished, I will play her a little and then determine if she is a good fit to replace one of my other decks. If so, I will take apart the deck she replaces. If not, I will take her apart. I plan on taking apart Mina and Denn right now as it is the least flavorful or unique deck in all the ones I have built. Prime Speaker is a potential cut as well.
I like to keep a maximum of 12 decks for a couple reasons. One is that I have it set up to be able to carry 12 decks, dice, tokens, and 2 playmats in my backpack. It works pretty well. I have a little space to carry an extra deck or two for early testing, but 12 is the right number. I also don't like having too many more as then it takes longer to rotate through the decks and I end up with decks I never play. That is the problem I had when I had upwards of 17 decks. Within the 12 decks I have, 3 are my "go to" decks: Alesha, Who Smiles at Death, Sidisi, Brood Tyrant, and Ephara, God of the Polis. These are the ones that get played the most often and I usually start a night out with either Alesha or Sidisi.
One is my more casual Garza Zol, Plague Queen deck and is meant mainly for unknown metas. It is meant to be lower powered so I don't come out swinging too fast if the meta isn't used to it and I would rather lose with an under-powered deck the first time in a new meta than crush with an overpowered one. It does have some power behind it, but it is a fun Vampire tribal deck that allows me to gauge the meta. I can then change the decks I play and the way I play them to suit the meta a little better. It is not played often due to not often playing in new locations, but I like having it just in case and it is a fun deck to end a night on sometimes.
My Narset, Enlightened Master deck exists solely to fight against players who insist on infinite combos or ways to win before turn 4. It is not a play style I enjoy so I don't play it often. but I keep it around just in case.
Otherwise, the rest are Thassa, God of the Sea. Gisela, Blade of Goldnight, Karametra, God of Harvests, and The Gitrog Monster. These have a decent power level and they are fun to play. I generally play Karametra and Gisela the most of these as they are the most fin to play against. Thassa is Mono-U control and can be frustrating to play against (but it is a play style I enjoy) so she isn't played frequently and The Gitrog is probably my favorite deck after Sidisi. The main problem with Gitrog is that even without infinite combos (I intentionally omitted Dakmor Salvage for example) the turns can often take a while to complete. I used to play in a "league" where we had timed rounds so I didn't want to take a bunch of time taking long turns. Now that I play in a different store where the games go as long as they need to, I might play this deck more often.
So I guess this is a long way to say that, for me, anything more than 12 decks is too many. Since 2 decks (Narset and Garza Zol) exists for fairly specific reasons, I guess I could say that a rotation of 10 decks is the max I want to go. After years of playing and being above and below this number, this so far has felt like the right number.