My limit is going to end up being storage space! There are only so many complete bikes which can fit in my basement workshop--- (or at least there's a point at which my wife's raised eyebrows will get very hard to withstand.....). Bare frames are a bit easier to store/hide, I guess....
My affection does not seem to have a limit. I try to ride every day, so I get to ride most of my collection at least a few times a month.
I agree with the poster who said that your pleasure in the hobby should be your guide--- it's still a pretty cheap/healthy way to enjoy your time on the planet. (Though I wish I didn't like expensive tires so much--- that's the part of trying to ride all my bikes that gets pretty pricy!)
Old cars are far worse on every front: storage, cost, price of part, difficulty of repair..... ask me how I know.
N