if i am using mikritok cHR with usermanager is it possible to run 20000 cuncorrent users
And what should we base our evaluation on?
That of course depends on your definition of “is it possible”. It may be possible when you have the proper hardware to store that many users, but undoubtedly you will run into issues when trying to manage them. E.g. when you open the “Users” tab in Winbox it may take a long time to load the list of users, it may be slow, it may consume a lot of memory, etc.
Also, when you have a system that large, you would likely want to have a backup server that replicates the same configuration and serves request when the primary one is down. User manager has no real support for that.
I would recomment a more professional RADIUS solution. Still, it might be “possible” to use user manager.