There are suggested guidelines for tank sizes. There are a variety of factors that go into these recommendations and you will often get mixed recommendations. Check out places like liveaquaria.com or blue zoo.com for a general idea.
A 10g saltwater tank would be difficult to maintain and keep all parameters in check. I wouldn't suggest it as a starter tank.
Are you talking freshwater betas? Are you taking male or females? Males need to be kept single, but yes, you can do multiple females together.