I want to say there are currently 4 as far as I know.
1. RS5 - found in 3 series.
2. RS7 - found in 7 series.
3. RS8 - these are relatively new and TBH I'm not sure they are actually called RS8s or what series they will be found in, but I know there are 8" drivers in the lineup.
4. AlNiCo Hemp Cone - found in AlNiCo XRS and AlNiCo monitors.
There used to be 3 versions of the RS7 at one time (RS7 = Regular, RS7A = AlNiCo and RS7V = Vintage), but I think the Vintage and AlNiCo were dropped due to price crowding and other factors.