I would say basically anywhere with a Pokemon Center is a different Town. I am doubting that they would have so many random Pokemon Centers scattered when they have had doctors. Not to mention it looks like the Poke Marts will be returning in Poke Centers, as I do not see consistent buildings that would be a Pokemon Mart in every town.
There is 3 to 4 on the starter Island. 2 Pokemon Centers. You have them circled, but there is another Pokemon center near the edge. of the big town.
Second Island, the top right, has about 6 pokemon centers. 2 right next to each other near the port building. The bigger town near the center of the island. One at the very top of the island, one a bit south of it, and another south of it.
The biggest island also has about 6. The gated town. 2 on the mountain, maybe one, the big town with the port on the right side. One near the beach/water route south of the gated city. Then 1 or 2 on the south side of the mountain.
And on the empty island where the legends will probably be, there is 1 town. It might just be a few buildings though.
Some of these could be 1 big town, but these are what I think are towns or villages. Some could be really small. It is really hard to tell where the routes are. Some are obvious, but there is so many buildings scattered that it is really hard to tell.
The orange circles are what could possibly be towns.