A MATLAB license is checked out when the application is executed/opened. MATLAB toolbox licenses are checked at a first reference to a component of a toolbox.
As a general rule, best practice is to checkout all required toolbox licenses soon after MATLAB startup - especially if running MATLAB jobs on the HPC compute nodes.
Officially, a MATLAB license is defined as: 1 user on 1 computer using no more than 2 CPU cores.
There are times when all licenses for a MATLAB toolbox are fully consumed. As a general rule, there is little that HPC staff can do to give you near-immediate access to a license in such situations.
I do not recall any time when all 50 MATLAB (base/core components) licenses were confirmed as checked out.
If HPC staff can verify numerous such events for a license, we can request that extra license(s) be purchased - may take days/weeks.