I think the word is urban sprawl or megalopolis...many cities growing and expanding so close to each other that they seem to form one huge city. Rural means low habitation as in woodland or farmland. As for ideas on a map like yours, just dots close to each other. Drawing each building is also pointless unless each city has a very unique architecture apart from the others...if they don't then the buildings will all look the same anyway and you are back at square one. You could try color variation or lines separating them but that probably won't look too good.