Yes, you need a minimum number. I would've thought the number to be about 6, though it might be seven if that's what the game is telling you. I play on large or huge maps, so founding the required number of cities is normally not a problem. There's no way to adjust it for map size or number of civs, either. The other early NW's also require this same amount of cities, so you'll need to get a couple more cities in your empire.