This number obviously varies greatly from person to person. People like Ben Hogan and Vijay Sing hit an amazing number of balls a day just to ensure they won’t lose the secret to their swing. Others feel like they need to play, and only play.
The best advice we can give to anyone is to only hit as many balls as you can while you are able to give it 100% focus. Once your focus wanes, take a break or go to the putting green. You will never be doing anything positive for yourself or your game without focus and a plan.