There isn’t really a maximum number of Members that WishList Member can handle because the number is based on server capabilities.
For example, with 10, 20, 30 thousand people you would probably feel some slow downs not because of the software but more because of the server.
You would need to isolate the database to a different server since there would be so many people logging in and out accessing the database.
The performance of your site with that many people will largely depend on how your server can handle the load. If it’s optimized, then you shouldn’t have a problem.