The number and type of RODI stages is really determined by the source water you're feeding into it, and what's in that water. I would first start with your municipal water supply's website, and see if they have information on the sanitizer they use (typically chlorine or chloramine) and anything else that's in your water.
That said, I would suggest getting at least 5 stages because usually with 5 stages you're getting two DI stages. Having two or even three DI stages allows for more efficient use of the DI media, which saves money over time.
Popular RODI systems include BRS, SpectraPure, and I'd suggest Buckeye Hydro who's a Reef2Reef sponsor, and available here on R2R to help and answer questions about their RODI systems if/when needed.
@Buckeye Hydro
Good luck!