If we're talking ramp spells, counterspells, mana rocks, these are the foundations of any solid build. You can't help doubling up, and you shouldn't walk the line just to prove a point. You're doing yourself, and your decks, a disservice if you do.
It doesn't even cost a significant amount more in most cases - unless you want all of your decks extremely well tuned.
For myself, I have a reasonable rares folder, and my aim is to use as much of it as possible and have it be relatively empty. Having 2 or 3 decks would not achieve that, and having that number of decks would not give me a Mishra's Workshop or ABU dual lands. On the other side of that argument, I could probably lose one or two decks and make a really sweet mono black reanimator, but I don't want to because I feel like most of the cards I have are in their rightful place.
I get the point being made, but I think 'work within your means' is what was meant. I'll gladly keep brewing all day long, but I do try to be rational about how many decks I have and how much time/money I'm investing in the game.
I have, at a guess, 18 decks. Some of them are pretty well tuned, and some are casual level.
I guess it depends on your goals; for myself, I'd like to have a deck for every occasion - I have combo ghave, durdly braids, a couple decks almost anyone could pick up and play, some are unpredictable and chaotic, others are more control based, some are reanimator themed, some are ETB, you get the idea.
For myself, with what is now a limited budget, my aim is to make the decks I have as good as I can without spending any further money on them; I'm hopefully buying a house soon, so cardboard sort of pales in comparison. Thinking this way kind of came from looking at my folder and spares boxes and seeing how many reasonable cards were not being used - so now I'm harder on myself - if I can't use it, lose it. The ideal for me now is to not build any further than I have, refine the decks as best I can, and strip back my collection to its bare bones otherwise.
I think though, if you're just building to theorycraft and not to actually play, or if it doesn't fill a perceived niche, then you have too many decks. That's a different number for everyone though.