The simple answer is to just add a line such as
Licenses=whatever:20
and then request your users to use the -L option as described at
This works very well, however it does not do enforcement like Slurm does with other resources. You will find posts in this list from me trying to achieve such enforcement with prolog, but I ended up banging too much my head on the keyboard and so I eventually gave up. User education was easier for me. Depending on your user community, banging your head on the keyboard might be easier than educating your users -- if so please share how you solve the issue