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