I usually put about 20 snails in every two weeks. Of course I have 6 dwarfs. So if I were you, I would start with a low number (maybe 5 or so) and go from there. See how long it takes them to eat those snails, and maybe add more or less.
My dwarf once ate 13 in one day (or more) but 2x a day sounds great if you add them slowly like that you will never need to fed it anything else (except on special occasions) I ran out of pond snails and cant go to the store for a while so I put my last one in a shot glass and buried the rim. my little guy can see him but cant figure out how to get him It's sad to watch but I know it will help him feel like he is hunting so I do it anyway.