Umm... I don't have an answer for you. But concurrent users is not really an exact figure.
Because the answer is. It Depends.
It depends on exactly what the users are doing.
What traffic is being inspected. What which inspections are configured?
What rules are firing. (iRules) and what are those rules doing?
Are connections persisted?
To get your answer, what you need to do is test. Run a defined number of users and check the resource usage (Memory, CPU, etc). And test at different numbers of users so you can see what the increment is per user.
And if you want to make out more efficient then you need to know exactly what fires for each 'user' and determine if there's a better way to do that.
Sorry, it's not a magic band-aid. But that's what you hire BigIP experts for right?