Generally ten or so is enough to keep me from getting bored on any one of them... and not too many so as to blur them all together.
We have 20 on the list to learn in the next two weeks... that's an excess to weed out ones that don't sound right. I make it a point to know the progression, general guitar work, and what the rhythm section is doing... and work out the kinks at practice... hehe or live.
Remember, wherever you go... there you are.