Something like a .223 that is cheap on plinking ammo and has a ton of components available for reloading would be a good start for target shooting. If this is a used rifle you are buying that someone else has "proofed" to consistently shoot .3moa find out what load recipe they are using and you should be at least close to good; unless the barrel is at the point where accuracy is starting to degrade. But to your original question how many trigger pulls? How many can you make in a day? Practice doesn't make perfect; perfect practice makes perfect. Make sure you practice well and don't just go blast ammo for the sake of trying to reach some magic number of rounds that will make you a better shooter. Spend a lot of time with that .22... More people have learned to shoot and shoot well on a .22 than anything else I'd wager; cheap to feed and no recoil so you don't develop a flinch.