Is GND isolation required to parallel a large number of power supplies
In general you do not need or want to isolate the grounds.

However you can't usually just parallel power supplies.  They will not share the load equally.

There are power supply module that are designed for being placed in parallel.  Often they are intended to not only increase output but allow "hot-swap" repair.  They are often referred to as being in an "N+1" load sharing configuration where there is one more power supply than is actually needed so that when one fails the remaining good ones can power the load while the failed one can be unplugged and replaced with a good one.  These are used large rack-based server systems and for telecom applications.

The power output and grounds of all the supplies are connected directly together and often there is a load sharing signal that also connects to all the supplies.

Why do you want to put many power supplies in parallel?