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It really depends on what your starting TDS is and what is in your water now. The more stages allow you to tailor the system to filter out more contaminants like chloramines, ammonia, and phosphates. You can start with a 4 stage if price is of concern and then add additional stages with different types of or larger quantities of media based on how well the system performs. You also want to make sure that you have adequate pressure to your RO membrane. Ideal pressure is typically in the 60-100psi range and will vary depending on your unit. Most municipal and well systems run at 50-60 psi, but can often dip into the low 40's at times. So the addition of a booster pump might be required to ensure the RO membranes are getting the correct pressure to operate as designed. The best overall suggestion is to get the one you are most comfortable with running taking maintenance into consideration and have your tap water analyzed before and after the unit so you can get an idea of how bad it is, how well the filter is working and what if anything still needs to be removed and then update the system from there.
I started with a 4 stage but ultimately upgraded it to 6 to run three different types of DI resins to remove the trace amounts of phosphates and ammonia that slipped past the rest of the filter stages.