I think those are two different questions actually. How many taps should I have and how many kegs should I have. If you have room in your keezer for 4 kegs I would do at least 3 taps, depending on the rest of your situation. I fit 4 kegs, I have 4 taps. But i have a fermentation fridge I can use to get/keep kegs cold. I have 5 total kegs and I'm finding i'm about 2-3 short. here's my math: 1-2 kegs of beer ready for when a keg kicks, 1 for sanitizer, 1 for PBW (both are used for cleaning lines and closed transfers)... maybe 1 more empty just in case you get the urge to brew again.
Also have back up kegs of beer (especially if you can keep it cold or cool it quickly) gives you the change to swap out styles based on what you feel like or what guests like. I have a cider I really only put on tap when we have cider fans over for example.
Just my 2 cents.
This ^
Something else you can consider is starting with fewer but leaving room and planning for more later. I wanted to use Perlick 650 SS flow control faucets on my keezer, so I started with 3. But I planned for expansion, both in buying a drip tray wide enough to accommodate as many as 5,
*and* I marked the collar where I would add the 4th and 5th faucets.
I laid out the spacing on the collar, using an awl to make a divot where I'd need to drill. I made tiny divots where the 4th and 5th would go if and when. You couldn't see them unless you knew where to look, and I could find them easily just using a ruler and extending the spacing from the other faucets.
BTW, drip trays are expensive, so don't space the faucets any wider than you need. Mine are spaced 3" apart on center, and with flow-control faucets I wouldn't go any closer together. If you don't use flow-control faucets you could go 2 1/2" apart. But whatever you do account for the black plastic trim pieces--2 1/2" is about the limit for narrowness of spacing.
I can get 6 kegs in my keezer, so 5 faucets seems like a reasonable number. I can keep an extra keg in there, or force carb a keg if I want. Lo and behold, as I brewed more beer, I needed a way to dispense it, so a 4th and eventually a 5th faucet ended up being added.
A friend of mine is adding faucets to his keezer; they should arrive today. His is a small keezer, he can only get 2 kegs in there, but he's going to use 2 faucets. He might be able to use a shorter keg on the hump, so I'm going to suggest he measure out and plan for 3 even though he's only installing two.
