Maximum number of users in AEM

There are many environmental factors to consider for this question such as the server running AEM, the RAM, etc. See this artilce: 

http://adobeaemclub.com/max-number-of-users-in-cq-publisher/

Hope this helps. 

There is no hard Limit as such.

Also please have a look at this forum post:- http://help-forums.adobe.com/content/adobeforums/en/experience-manager-forum/adobe-experience-manager.topic.html/forum__ixpv-how_many_application.html

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Question: We are planning to have a CQ publisher around 65k users & around thousand groups. Will there be any performance issues & do we have any limit on number of users you can have in publisher.

Answer: There is no restriction as such. But if you have your users already outside, you can also have your own external login module for authentication and authorization. You will, however, want 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 didnt call this out specifically, but if you have tens of thousands of users, odds are you have some big groups 

I hope this will help you.

~kautuk