Captive Portal is part of Identity Awareness, and the above only applies to Remote Access VPN.
While we have mechanisms to filter out users that appear on multiple computers, that requires R81.20.
This doesn't "restrict" a user to, say, 5 logins, but it invalidates ALL sessions for any user that exceeds whatever you've configured the threshholds for.
This is also not the specific use case for this feature (it's designed for Service accounts specifically), so it may not work for that purpose.
Assuming you're using an external identity source like Active Directly, it should be possible to configure such login limits there.
For locally defined users where the password is defined in the Check Point management, there is no way to prevent them from logging in multiple times; this would be an RFE.