Privilege Manager does not have any specific built-in functions or controls for monitoring and limiting the number of sessions a user is running, the privilege manager policy is quite flexible and its conceivable that this could be implemented using external commands and the system() policy function.
In order to determine the number of sessions a user already has running currently, iologging would need to be enabled for all sessions, you could then count the number of iolog files that are currently being written to by pmmasterd processes for the specified user (using lsof, grep, and wc). This would have to be done for all policy servers in the policy group.
Once you have an external command that is able to determine the number of sessions, the policy can be written to run this command via the system() function, and compare this to their pre-determined limit to decide whether the new request is accepted or rejected.