You can't encourage the largest number of NPCs to visit your town because they have the same chance of visiting your towns, as far as I know. The only thing that will give you more NPCs is the campsite (where you'll get campers) but that's only about one per week, and if a few of your villagers have moved out, they'll show up on your main street. With the rest of the NPCs, it's random and not really based on your town's design.
Maybe I'm misunderstanding what you mean, which NPCs are you talking about? Villagers? Special characters? Other human visitors and side characters?
Joan is Sundays from 6:00am to 12:00pm
Redd, Saharah, and Gulliver are randomly on one day from Monday-Saturday (Redd also shows up during New Year's Eva and the Fireworks show, but its not with his usual wares)
Katrina is also randomly on one day from Monday-Saturday, until she offers to stay in your town (afetr you get 20 fotunes told iirc)
Gracie shows up randomly after you unlock T.I.Y. and passing her tests multiple times is part of the requirement for unlocking the Emporium
Pete fills in the empty slots for the week
Katie is random, but shows up more often if you connect with other people to visit/be visited
Phineas shows up when he has a badge to give you
Leif shows up (outside of his store) on Weeding Day and when your town has 50+ weeds
Pascal shows up if you catch a scallop, and tries to trade you it for a piece of pirate furniture
Dr. Shrunk shows up wen he requests you to petition for Club LOL, and afterwards he will be there on all days
K.K. Slider shows up on all nights at Club LOL once you build it, with his usual jam sessions being on Saturdays as always
Brewster shows up working at The Roost Cafe once you build it
Celeste shows up at the 2F of the museum, running the shop and watching over sleeping near your personal exhibits
Resetti shows up if you reset, and if you build the Reset Center you can see him and his brother Don in their workplace/home(?) during evenings on days after it rains
Tortimer shows up after you get you first house upgrade (so the upgrade after upgrading the tent) inviting you to his private resort (with Kapp'n taking you there for a 1000 bell fare each trip to and back), and afterwards runs the tours you can go on at the island
Wendell shows up in dream towns to give you designs he stole