What are the maximum values for
$MaxLicenseSubprocesses? Can they be infinite or are there specific limitations? Does
$ProcessorCount depend on the license or on the machine?
$ProcessorCount tells you how many CPU cores your machine has. It characterizes the hardware and is unrelated to licenses. This is how many subkernels are launched automatically by default, but you can launch manually as many or as few as your license allows.
$MaxLicenseProcesses tells you how many main kernels your are allowed to run at the same time. This is basically how many separate instances of Mathematica you can use at the same time on the same computer. It is not relevant for parallelization.
$MaxLicenseSubprocesses tells you how many subkernels you can run at the same time. This is relevant only if you use parallelization.