There is no maximum number of members when using WishList Member. A site running WishList Member can have unlimited members so you can include as few or as many members as you would like within your site. This means you aren't limited by a maximum number of memberships you can offer.
Something to keep in mind is a large number of members (tens of thousands) could possibly contribute to some potential site slow down but this would depend on the server capabilities of the hosting account being used.
Hosting / Server Environment
As an example, a site with 10,000 users might experience some slow periods if the server isn't capable of handling that type of traffic. With that many users, it is recommended to isolate the database to a different server since there would be so many users logging in and out on a consistent basis and accessing the database.
The performance of your site with that number of users will largely depend on how your server is equipped to handle the load. If the site environment is optimized, you should not run into any issues and your site can handle as many users as you'd like.
