You can connect up to 3 RS cabinets to an amp. You can connect any combination of RS210 and RS212.
Conventional wisdom would tell you, that since the cabinets are 8 ohm and the amp is rated at 4 ohm you would only be able to connect 2 cabinets. However, a speakers impedance varies depending on the specific frequency. A speaker might be 4 ohms in the lower range but could at the same time be 8 ohms above 10Khz. So a speakers rated impedance is an average across the spectrum. Since the speakers in the RS cabinets are custom made we know the exact Ohm rating of the them, and have therefore been able to design the amp so that it can drive 3 of the RS cabinets.