Show the default value chosen for XX:ParallelGCThreads

Default value :

 java -XX:+PrintFlagsFinal | grep ParallelGCThreads
 uint  ParallelGCThreads                        = 4

If you have a running process jinfo <processId>, if it's not present in the output, it is using the default value (look under VM Flags)

Consider the default is a fixed number N, as in -XX:ParallelGCThreads=<N>

As per the documentation, that number is based on the amount of HW threads your system has:

On a machine with N hardware threads where N is greater than 8, the parallel collector uses a fixed fraction of N as the number of garbage collector threads. The fraction is approximately 5/8 for large values of N. At values of N below 8, the number used is N.