I probably keep 10-15 at a time.
But knowing the basic details unique to that card can be helpful - category bonuses, priority boarding, free checked bags, etc. I also sometimes choose which card I use when buying something big since some extend the return period or double the warranty period.

But there are all those other things like travel insurance, trip protection, car insurance, etc. A lot of it is probably the same for "all signature visa cards" or "all world master cards" or something.