The JVM maximum is 8 PGC threads and 1/4 of that for ConcGC.
This is not correct. If there are more than eight CPUs, the default becomes 5/8 of the number of CPUs rounded up to the nearest even number (it seems - see below). See -XX:ParallelGCThreads=n secion of http://www.oracle.com/technetwork/articles/java/g1gc-1984535.html
Pretty easy to test with > 16 cores (as 5/8 of such is 10): turn on GC logging, leave the defaults, and the G1GC output will show something like:
on a 24 core system in this case.