The formula to calculate the pg_num on Proxmox is as followed:
(OSDs * 100)
Total PGs = ------------
Replicas
If you decide to increase the amount of disks, where the pg_num value will increase, you also have to alter the value of pgp_num to the same value as pg_num, to restore a healthy array:
ceph osd pool set {pool-name} pg_num {pg_num}
and
ceph osd pool set {pool-name} pgp_num {pgp_num}