There is no maximum number of users in the specs. As long as your license supports unlimited number of users you can have as many as you can making sure the ASA is not oversubscrbed.
For example you could have 10K users that are just browsing and the ASA5520 would be able to handle the traffic. You could have 10 that are passing streaming media at high speeds which would oversubscribe it. So it is not a matter of number of users.
From experience I would tend to think that 5.3K users would be a lot for the 5520, unless they don't generate much traffic though it, or a re idle all the time. But again, it all depends on what the users are doing network-wise.
I hope it helps.
PK
