Limiting connections based on number of users to a backend server

I would like to know if it is possible to limit the connections to a backend server based on the number of concurrent users.

 

 

For example i would like to have not more that 300 users being sent to a backend server at a particular instance of time.

 

 

I understand that we can limit based on connections but is it possible to limit based on users? If YES please let me know how this can be achieved?