I’m asking because I used to run into the maximum number of blocks on my Reddit client
I think they used the multi-reddit list as the back-end for subreddit block list. I too ran into the limits.
I don't think Lemmy has any limit, but performance will likely degrade and it is entirely possible that your personal block list causes overload of servers with lots of data in them.
Perhaps you are unaware of just how unstable Lemmy has been since late May.