Ok, I thought of a few other explanations if you are sure you still have as many snails as you purchased. One is that you had hitch-hiker snails (I have found at least 3 of these in my tank) and so even though some died, you can still count the same number of snails as you purchased. The other is if you bought live sand, it is possible that the extra empty shells were in the LS, and just surfaced. I ordered sand from SWF, and shells keep popping up out of the sand. Oh, and one other possibility is that if you have hermits, it could be that it is a hermit crab that died and/or left the shell instead of a snail.