bettas will be just fine with rasboras, the number you get all depends on the size of your tank. i dont know much about rasboras so you should probably ask on the appropriate forum or research them
I've never had problems with Bettas and Rasboras. The Rasboras aren't overly active, so they shouldn't stress the Betta out. Nor do they tend to nip at the Betta. So it seems to be a good combination. However, before you think about getting more fish...how is your cycling going? What were your latest test results?