Each pool has a fixed number of Placement Groups (PGs) *and* a fixed number of
OSDs. The former is set when the pool is created and the latter is determined
by the CRUSH map.
The number of PGs in a pool can be increased, but it cannot easily be decreased
(there is always the option to create a new pool, copy the data over, and
destroy the old pool).
If there are too few PGs per OSD, or too many, cluster performance will suffer.
In general, the number should be between 30 and 300 and the oft-quoted "rule of
thumb" for ideal performance is _approximately_ 100 PGs per OSD.
Since any given OSD may of course belong to several pools, depending on the
cluster topography and the CRUSH map, it's not easy to generalize about what
is the optimal/right number of PGs for an arbitrary pool. Still, the
Ceph PGs per Pool Calculator can help.