Depends on what you want out of your printer. 5 is the minimum for me for multicolor prints. I usually max out my MMU when I'm doing that, and the new color mixing can take advantage of even more.

3 is probably all you need for structural stuff. One PLA, one PETG, one TPU and you can make tons of very unique, useful items. But 5 colors + support + TPU? That's an amazing amount of flexibility and design options.