You can build as many as you like, but, the maps I've played so far are rather small.. also the 'enemy' is possibly very close. Since I try to use a growth strategy (big city vs. a lot of smal cities) I often do not end with that many cities, last time only 4 or 5 before I started my domination conquest (ending up with +/- 15 cities).
BTW...I haven't played civ for some time... I believe it was possible to pillage cities right? So you didn't need to make a conquered city your own? It seems civ-rev doesn't offer this possibility