Well for sessions you need D and G. Sometimes A, C or F, but not often. For soloing you could get away with one. For playing fills when singing - probably on a rack while playing guitar - you need as many as the keys you'll be singing in. Hence I have some "bizarre" keys such as a B cos that's the key I like to sing a couple of particular songs in. For accompanying others, then you'll need as many as keys they might choose i.e. 12.

...or, then again, the tunes may be using a melodic minor - which is posible but messy to spoof by bending the necessary notes - in which case you'll need 12 melodic minor specials as well.

So, in short, the answer is "it depends..."