I started with a 4-stage system and then bought another 3-cannister bracket. I have chloramine so I have to use two chloramine-specific cartridges behind the prefilter cartridge. Then it goes to the 100 gpd RO membrane and then to 3 DI cartridges.
Over the years, I found that one carbon cartridge will let chloramine pass. Any carbon cartridges will let ammonia pass hence the need for DI. Even with a modest 100 gpd RO membrane, the dwell time is such that a single DI cartridge won't remove all the ammonia. There's also the phenomenon where an exhausted DI cartridge dumps the ions it's previously bound, potentially leading to a release of ammonia in my case. The 2nd and 3rd DI cartridges take care of that and ensure that the 1st cartridge is fully exhausted before it's replaced. I have an inline TDS meter after the RO and after the 1st cartridge. TDS is 3 before and 1 after the first cartridge even though a lot of the color has changed. The 2nd cartridge has started changing color which shows that stuff is still getting by even though the 1st cartridge isn't completely exhausted yet. The 3rd cartridge hasn't shown any color change.
Water from the RO/DI filters goes into a 55 gal water drum where it sits for a few days while I adjust pH with baking soda, TDS with Kent RO Right, and warm it up with a heater, if necessary.