As always the amount to store depends on how much you practice and/or compete with it. The clay sports can eat up 100 shells quickly. Hunting needs depend on the game and limits. And of course sports like 3gun and Cowboy can also eat up lots if shells. For high volume use you may want 1,000 shells stocked up.
For more casual and less frequent use I have near the same as you. About 300 rds buckshot, 250 slugs, and 800 various hunting and target shells.