if your tank has been set up for about 3 weeks or more then you can get some snails.. anything nder 3 weeks seems to either not have enough food, or they could die during the main part of the cycle.
If the tank in Question is the 72 bow then i would say no more than like 7-10 snails of a mixed variety... they will soon run out of food to eat and die if you get anymore.. as it is i have 20 in my 55 and they seem to have exhausted the food in about a week... i have heard of people feeding them pellets, but i have never tried..
I would get a couple turbos, a couple margaritas and a couple smaller snails that tend to sift the sand...
you can not "make food" for the snails by overfeeding, so choose your numbers wisely.. you will discover that they have an enormous appetite..
HTH