depends how big your tank is!
but generally sea snails don't need much room, actually they don't need any at all! also depends on snails you have...
the ones i'm thinking of you can have 50 - 70 in a "med" fish bowl!
for a tank... 100s! really!
fish then you need to work out the same needed, esp without a filter!~ just change your water for the snails and you can have as many as possile!
umm... i've never seen snails have babies! if thats what you want! then again, i never had snails for years now!
any more problems ask Dr Harry Copper.. is that his name?