Max number of users in CQ Publisher

There is no restriction on number of users in publisher. But if you have your users already outside, you can also have your own external login module for authentication and authorization. However, you need to ensure you are using AEM 6.0 or higher and Oak. With Jackrabbit/CRX 2, such scale was possible, but you do need to do some extra configuration if you have groups with thousands of members. Your post didn’t call this out specifically, but if you have tens of thousands of users, odds are you have some big groups