If we base our speculations of towns off of the old clay model map we've seen in the past, there are quite a few number of towns in the game. As a few people have mentioned, there is a lot more to this game than a lot of us have seen so far. We've really only seen the areas that are close to the beginning of the game, just think about that...
Duscae region with Lestallum, Caem, Hammerhead, and the areas surrounding that are just one part of the region of this world. We have Accordo which holds Altissia and perhaps a few other areas, then there's Tenebrae, Solheim and Niflheim we have yet to see, we've seen a small glimpse of Tenebrae but not much. We know there is a volcano as hinted at when mentioning where Ifrit might be, as well as shown directly in the Type-F footage with the Regalia flying around different regions, you can imagine that the volcano will be part of a region that has many towns as well.
Also, remember, the game will go from more realistic in terms of setting to the fantastical as the game progresses, so we can expect to see even more crazy towns and areas in the game as we progress. Also, keep in mind, the game's story content is expected to be around 45~ hours, so you can guess how many towns a game like this would have in it.
I think we can expect quite a few towns and regions in this game, a lot of them will probably be around the size of Lestallum which is actually larger than you might think, the metal foundry like area was shown in the World of Wonder trailer, and that place looks not only important for the story, but actually huge.
And, don't forget about that massive train station we saw a long time ago, that place is enormous, imagine what sort of a town would need to accommodate a train station that size.