MSO you ask how many impellers you need, the answer
is just one...at a time.
You will be well served by keeping a spare on the boat, as these little buggers sometimes have a way of disintegrating at the most inopportune times!
Don't buy spare until you install and know the new one is correct
size as the GM engine series has several different water pumps
different size impellers.
Assuming boat is new to you, so suggest that you might change
belts as well and keep old ones as spares.
Had both an impeller go and water pump
belt break on same cruise
few years ago, with spares we didn't miss a beat and very
reassuring being self sufficient.