I'm curious, too, so I'll bump the thread. Maybe Travis or a mod will respond. (Although I think 10,000 might be a bit excessive.)

I saw the thread a few days ago, but I have no idea.

I believe there is no theoretical limit to the number of rows in a SQL database table. I know there are some databases that have billions of rows. The practical limit is determined by the amount of storage that is available on the server. I doubt 10,000 rows in a list would be a problem, and I doubt you'd ever need that many. But I don't know anything about the TMDb SQL database. Maybe Travis has put some practical limits on user lists, but he may consider that information to be proprietary, not public.