I usually try to get 6 cities up as fast as I can, in the best locations. Strategic resources are the most important to get claims on (Easier if you're Creative/have built Stonehenge; also, they provide HAMMERS. :3). After that, I try to get food resources, and Libraries, if there is no one close-by to conquer. I launch early wars, keeping maybe one or two of my enemies cities at the max.
By Currency/CoL, I usually have 10-15 cities. One time I had 25, and my science rate was 0%, and I had negative GPT. This was around AD900, and I had JUST gotten Currency (I was playing as Rome, okay, so I HAD to go kill everyone). Not a good idea. I whipped courthouses, but I had no science. At all. And the ever-present war weariness.
So, yes, try to stay under 7 cities pre-CoL/Currency, under 15 post-CoL. If you're financial and have coastal cities, then you can sneak one or two extra cities in.