The limitation is for users with access to the admin control panel. They include Writers (Authors), Editors, and Superusers. The license restriction is on the number of admin users (staff logins) that can be created to log in and update your knowledge base. These are the users who must be licensed. For example, you can have 1,000 users with a PHPKB account, but if only five of them have a privilege then you have five Admin users, and your license only needs to cover these five. There is no limit on the number of end-users to the knowledge base.