Depends on the pool. When I worked at a swimming pool store, we saw all sizes, 10-15k gallons were commonplace.
Here's the easiest way to calculate it:
SHALLOW END DEPTH + DEEP END DEPTH + 2 = AVERAGE DEPTH
LENGTH X WIDTH X AVERAGE DEPTH X 7.5 = TOTAL GALLONS
CIRCULAR DIAMETER X RADIUS X AVERAGE DEPTH X 5.9 = TOTAL GALLONS
FULL WIDTH X FULL LENGTH X AVERAGE DEPTH X 5.9 = TOTAL GALLONS
Free Form Pools: Check with builder.