Check if the following VMware article helps:
Operational Limits of Site Recovery Manager
There is no issue with creating 450 VM's under one protection group it is definitely supported. But not advisable, the reason being. If one datastore is effected it impacts the entire recovery for that protection group. I would say create a group of 3 PG's based on their dependencies like APP/DB and that should work out as expected.
Regards,
Ritesh