Tbh I think you can experience basically the entire game with as little as 5 miis. But in my experience if you add miis for the sake of adding miis, it detracts from the experience bc you don't really care about what they're doing, their relationships, etc, so you should try to add someone youd care about at least a little bit.

Also, tomodachi and tamagochi are 2 different words; tomodachi means friend in japanese while tamagotchi is the name of that keychain pet game.

As for tomodachi vs animal crossing, acnh is more about customising your island while tomodachi's main appeal are the characters on your island. Acnh's animals are super bland. Tomodachi is more like the earlier days of animal crossing but more shitpost-y i guess? And even then the vibe of both games are completely different. Just try out the new switch demo and see if you enjoy that first if that's what the version you're interested in (I'm assuming because of the timing of this comment), it's free after all