wow how many books do you need for all those events?
Most Release Challenges don't have any minimum amount of books you have to release to play at all. A few do--the Oh, the Places We Can Go Release Challenge has a minimum of 15 I think, and the You're Such an Animal Challenge has 20. There may be others as well. But of course even for those Challenges people sometimes sign up and start releasing and find they may not have the minimum amount. Most are just release as many as you want to.
I happen to be addicted to BookCrossing and release a lot of books and, even though most people don't join in Release Challenges at all, they are what I enjoy most. I'd say 99.5 % of my releases are for one or multiple Challenges. But that's just me. For me that's what makes releasing fun.
Most Challenges you can join in if you have one book to release, or 5 or 10 or any number. Here's the list of all the ones that have been announced for this year, so far, although others may pop up later. http://www.bookcrossing.com/---/537362
The rules for each Challenge are set by the person who started it and all are different. Look them over and give some a try if you want to. You might become hooked too. : )
ETA: here's the Release Challenges forum if you want to explore last year's Challenge rules to see which ones might interest you to join this year. (There are also some links in the thread I posted above). http://www.bookcrossing.com/---/23
ETA: my addiction isn't getting any better. I came home with 187 books for assorted Challenges from a local library sale on Saturday. And there's another one the upcoming Sunday.