what is the maximum number of trolls you can have on your block list?

over 9k on normal games and 1337 on ranked games

Why would you block trolls anyway, they add you everytime to their friendlist or something ?

The problem is not going to be reaching the limit, it's going to come from the logistics involved in scrolling through those names during champ select.

There is still indicator there that you have blocked someone. The block list has no method, other than scrolling, to reach a particular name.