The question should be more like; How many commander decks can you actively maintain?
Some people build a deck to reach a specific function, and once they are happy with it, either sit it aside or scrap it for value cards so they can work on their next project.
The trick with running multiple EDH decks (when you start worrying about having too many), is when it comes to maintaining all of the decks simultaneously. Keeping up on actually "completing" the deck to what you want it to be, optimizing, updating with new cards, etc. The more decks you have, the more difficult that becomes.
Optimizing multiple decks is also very financially straining. For example, are you interested in making a competitive deck? Does that mean you need a mana crypt? Wouldn't that cause your play-group to eventually become more competitive due to the nature of your expensive mana crypt inclusion? Would you not then feel like your other decks need a mana crypt also? That one single card can make a huge difference in a decks performance. There are many single high-value cards like mana crypt that, when putting in all of your EDH decks, can really destroy your wallet.
If you are wanting to have a deck of every color, or a deck that fulfills the common deck-archtypes, and you have the resources to build it, then go for it. If by making another deck, your current 4x EDH decks may fall behind in optimization/power/completion, then you may not want to start a 5'th project.
I personally own far too many EDH decks. Past the normal 8x EDH decks I carry around (1x of every color combination of Grixis + 1x mystery deck), I have 9x in storage that don't see hardly any play (deck-turned-collection or became too financially valuable, an older deck/general with an outdated concept, something too weak due to power-creep that I don't like to see loose, or something nostalgic). I also keep 2x (budget) decks on my desk at work, and 3x decks are always kept at my apartment (used for roll-playing, have some theme or restriction).
When I realized how many I owned (22x), and that I kept building/stripping/building/re-building/etc, I decided to keep my collection at this exact # of decks. I like the grixis colors (hate green/white), so in my main 8x decks, I just keep a deck active per each possible color combination. The mystery deck is a green/white one, that I maintain simply to own a deck to pile in green/white valuable cards I trade for (and sadly while I hate the colors, it has become one of my stronger decks).
I may eventually fiddle with the two I keep at work, or the 3x theme/roll-playing decks, but those are not very significant in value, so I'm not too worried about them. The 9x "retired" decks will always be kept.
Some people build a deck to reach a specific function, and once they are happy with it, either sit it aside or scrap it for value cards so they can work on their next project.
The trick with running multiple EDH decks (when you start worrying about having too many), is when it comes to maintaining all of the decks simultaneously. Keeping up on actually "completing" the deck to what you want it to be, optimizing, updating with new cards, etc. The more decks you have, the more difficult that becomes.
Optimizing multiple decks is also very financially straining. For example, are you interested in making a competitive deck? Does that mean you need a mana crypt? Wouldn't that cause your play-group to eventually become more competitive due to the nature of your expensive mana crypt inclusion? Would you not then feel like your other decks need a mana crypt also? That one single card can make a huge difference in a decks performance. There are many single high-value cards like mana crypt that, when putting in all of your EDH decks, can really destroy your wallet.
If you are wanting to have a deck of every color, or a deck that fulfills the common deck-archtypes, and you have the resources to build it, then go for it. If by making another deck, your current 4x EDH decks may fall behind in optimization/power/completion, then you may not want to start a 5'th project.
I personally own far too many EDH decks. Past the normal 8x EDH decks I carry around (1x of every color combination of Grixis + 1x mystery deck), I have 9x in storage that don't see hardly any play (deck-turned-collection or became too financially valuable, an older deck/general with an outdated concept, something too weak due to power-creep that I don't like to see loose, or something nostalgic). I also keep 2x (budget) decks on my desk at work, and 3x decks are always kept at my apartment (used for roll-playing, have some theme or restriction).
When I realized how many I owned (22x), and that I kept building/stripping/building/re-building/etc, I decided to keep my collection at this exact # of decks. I like the grixis colors (hate green/white), so in my main 8x decks, I just keep a deck active per each possible color combination. The mystery deck is a green/white one, that I maintain simply to own a deck to pile in green/white valuable cards I trade for (and sadly while I hate the colors, it has become one of my stronger decks).
I may eventually fiddle with the two I keep at work, or the 3x theme/roll-playing decks, but those are not very significant in value, so I'm not too worried about them. The 9x "retired" decks will always be kept.
Links to my most current deck lists;
Primary EDH; Rakka Mar Token Perfection, Crosis Mnemonic Betrayal, Cromat Villainous, Judith Gravestorm, Rakdos Empty Storm, Exava Artifacts, Bant Trash, & Fumiko Voltron!
EDH kept at home; Ruzzian Isset & Rakdos LoR!
EDH (nostalgic/pimp/retired) in storage;
Latulla Burns, Akroma Smash, Jeska Voltron, Rakdos Storm, Bladewing Darghans, Lyzolda Worldgorger, Xantcha Steals your Heart, Jori Storm, Wydwen Permission, Gwendlyn Paradox, Jeleva Warps, & Sigarda Brick!
Legacy Showanimator and High Tide!