Your needs will depend on the quality of your tap water. Here in Stoon I get a TDS of about 180ppm from the tap so I use a full 6 stage RO/DI to bring this down to the range of 0-4ppm. I've talked a few people down in BC who get a reading as low as 20-30ppm so they can usually just use a dual or triple stage (no ro or di) and get similar results as me.
RO units are very wasteful and I don't believe you should use one unless you need to.
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