If you mean up to 50 provinces, then 1, as the Julii which was my first. I'm very close to that on my Gaul camp, but i play others now. Anyway, as most other ppl have already said, it becomes boring once you beat your No.1 enemy and sometimes not even that. Let's consider the Julii, you only need to take patavium and mediolanum and you can consider it a done deal (yes, segesta and segestica are close enough and segesta is a turn 1 senate mission, but that doesn't really matter).

Thats what i found Shogun to excell at, cos after you conquered half of japan, you found the other half owned (mainly at least) by some other faction which also had a lot of armies, so that kept thngs going.