Most hydraulic valves are rated for 11 to 13-gpm. You can buy bigger ones, but the. 11 to 13 are most common.
As others have pointed out it really depends on what you want to do. 5 to 7-gpm will be fine for most things involving cylinders. if you get into things with hydraulic motors or vibrators, the demand goes up fast.
However, if you wanted to run a hydraulic driven snowblower, mounted in the front, instead of the rear, you could mount a hydraulic pump on the PTO and plumb it forward to the hydraulic motor on the snowblower.
There are work arounds. Tractor Time with Tim, installed a higher volume pump in the JD 1025, he is experimenting (playing around), whitch also has a turbo which got a 50% gain in PTO horsepower. But that is a JD1025, which is kind of the 10-22 of the tractor world, just about every second vendor, custom parts source have things to bolt onto the 1025, which most other brands are impossible to find. But as tractors get older, less folks have aftermarket offerings made to fit it.