Depends, I'd started out my 2g with 6, three males and three females. It's pretty easy sexing RCS, females are bigger and redder while males are smaller and paler. Try to also aim for berried females, you can easily see them carrying a little pouch with dots, which are eggs. So I'd suggest you start with just six if you're tight on money, if you aren't you can start with ten or so.
It takes around 30 days for a new batch of RCS to hatch, so every month or so you'll be receiving an early Christmas gift.