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I have 6 in a 8g betta tank, so I'd imagine you could get a 6 or 12 to start and see what happens. Thing with ghost shrimp is if there's fish that can fit them into their mouth, they'll eat them from what I've heard. But I get them for .08 each, so it's not that big a loss.
I had 3 snails in my 29g and kept the same number for my 37g tall.