There is no simple optimal number of cities. For most parts I'd say the more the better, but there's always the maintenance expense, so sometimes it's worth it to stop expanding and build workers and infrastructure. And workers. There's never too many workers.
For domination victory you need some 65% or so land and population. That doesn't require killing off everyone - just expanding to cover most of the land there is. Usually this happens by killing off most of the others.. Conquest is where no other civ is left standing. Or in Warlords, no other civ is left standing as a free civ (you can win conquest by vassalling the others).
Then there is ther percentage issue.. Often it's said that at 60% you want to expand and at 40% you want to consolidate (percentages may wary depending on person). However, it all comes down to how many beakers and gold you can get total. If you can get 60% out of 100 commerce while breaking even, it's not really any better than 30% out of 200 commerce - both will give you 60 beakers. But at 30% you probably have more chances at improving the land and cities, taking the slider up to 40% soon, already better than 60 beakers.
I'm trying to learn to expand and wage war more. And in trying to learn this, I've had slider at 20% while building more maces, slider at 10% while taking more cities, but eventually slider at 0% and I end up having to stop. Or at least stop before I run out of gold.. At times it's better to push forward until you really run out of steam.