Please refer to this is post:- http://help-forums.adobe.com/content/adobeforums/en/experience-manager-forum/aem-communities.topic.h...
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As you've noted, to date, Adobe has only tested with 1M users on publish.
You mention avoiding creation of AEM users, but that Communities doesn't allow override of "that method/behavior".
What is it you want to override?
It sounds like you fear unlimited user registration and growth to 50M users, but there are configuraiton settings which allow you to make registration private and restricting creation of Community groups to privileged members (see Communities Sites console settings and Managing Users and User Groups).
