The factors which influence the outcome of a chiefing attempt are population difference between the two accounts, race and whether or not you have big party running in the village.
The strength of the chiefs/senator vary by race (from travian answers):
- The Senator (Roman) is very eloquent and persuasive; he can lower the loyalty by 20% to 30% each time.
- The Chief (Teuton) is especially cheap; he can lower the loyalty by 20 to 25%.
- The Chieftain (Gauls) is 25% faster than the others; he can lower the loyalty by 20 to 25%.
Big party gives +5% strength when attacking and -5% if the defender has big party running.
You can use kirilloid's conquering calculator to calculate how many chiefs you need to conquer a village: Travian :: Conquering calculator