Ghost shrimp have a very low bio-load, the only ones in your tank that are really effecting the load would be the snail (which has a very high bio-load) and the betta. I'd only add 2-3 more, because any more and you'll be pushing the limits on ghosties. You don't want to have too many or else it won't be the bio-load issue but space.
I hope that helps!
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