In the future(if they havent started to already), I think Google is going to be cracking down a bit on websites that are structured like this... Having X amount of pages for the same thing just changing it up a bit to target a new city.
If you have an office or address in those 20 cities, I would start with optimizing any Google+ Local listings and focusing on ranking those first. It's a much easier and legitimate strategy then making a bunch of spammy(most likely) looking pages.