To start this off, I honestly don't suggest having shrimp in tanks with other fish unless the fish are small. Like fry of other fish, or some sp. of tetra and other small fish.
Since you seem like your not having any problems in there, you could add prob. add atleast 20 more shrimp if you want too. Shrimp have very very small effect on the bioload that effects your filter, also they are small. Breeders of shrimp (like myself) can keep up to 300 shrimp in a 10G tank, but this is for breeder purpose only, since you just have shrimp in there to be part of the aquarium, then I suggest 10-20 more should be alright.
I just wouldn't suggest getting any other shrimp besides ghost shrimp. They are cheap and you don't have to worry about them getting eaten and having to replace them, because any fish would be happy to get a shrimp snack, and i'm sure your betta can pick at the shrimp/eat them easily. Unless you want to pay $1.50 per rcs and have them just be expensive snacks go ahead. If you want to keep shrimp, I suggest a shrimp only tank.