Perforce servers are licensed according to how many users they support.
Licensing information is contained in a file called license in the server root directory.
The license file is a plain text file supplied by Perforce Software. Without the license
file, the Perforce server limits itself to two users and two client workspaces.

To view current licensing information, invoke p4d -V from the server root directory
where the license file resides, or by specifying the server root directory either on the
command line (p4d -V -r server_root) or in the P4ROOT environment variable.

If the server is running, you can also use p4 info to view your licensing information.
The server version is also displayed when invoking p4d -V or p4 -V.