If you went with something like cherry shrimp you could probably have like 50, though if you betta decided they were a snack that would be more expensive... lol. It depends on how planted your tank is, but yeah, as long as your shrimp and betta get along you could probably put more shrimp in than you think.
You could also get shrimp and a snail, but I am of the understanding that snails are pretty big in the bioload category, particularly apples, the only ones the big chain stores seem to sell.
Remember that the tank your fish lives in is his or her's whole world. Ask yourself this question, and answer it honestly: would you choose to live there?