Sorry, I should have read the man page first.
For maximum cpu*time I'd try:
GrpTRESMins=<TRES=max TRES minutes,...>
The total number of TRES minutes that can possibly be used by
past, present and future jobs running from this association and
its children.
Assuming something like GresTypes=gpu in slurm.conf, I think
GrpTres=gpu=16 would limit a max of 16 GPUs being used.
