Unless you are shooting on a black target, any (decent) rifle scope with I'd say, 6x or more is going to show you your shot holes. You definitely will see the holes at 10x and above. The rest is pretty much as Graybeard said. A lot of guys are quick to skimp on spotting scopes, even more so than rifle scopes. I'll say that for sure that a "wallyworld special" $100 spotting scope won't pick up near as much as a good quality one will.
Personally I'd look into something with a range of 10 to 15 power on the lower end and 30 to 60 power top end. I used a fixed 20x spotting scope in the Corps in matches with good results.