I wouldn't worry too much about how many stages a system has - at least don't use that as your first criteria.
A four stage rodi consisting of (in order of water flow) a sediment filter, a carbon block, a ro membrane, and a deionization resin stage will do the trick just fine in most situations.
You can find many 6 stage systems on the market that are retro-fitted drinking water systems that are either mis-configured or have some other odd characteristics (e.g., horizontal DI stage).
Some things to look for:
Standard-sized prefilters, membrane, and DI cartridge
Brand name, high-rejection membrane
Specifications provided for each stage
Pressure gauge after the prefilters and before the membrane
Thermometer
DI bypass plumbing
Vertical DI stage
Refillable DI cartridge
Aluminum bracket
Quick connect fittings
Membrane flush valve
Clear housings
High-quality instructions
Customer support before, during, and after your purchase
A vendor involved in your hobby
Russ