8 characters per party.
I just started a temp party to test the viability of a 5 dwarf priest start. Started with max con+div, and the rest of the points in agi for defence. First thing I did was buy 2xchainmail + a longsword and two cloth helms. Put the armored priests in the front, and the others in the back. Paced out the starting offensive spells and used the town healer once spells ran out. They were managing to knock out stacks of 7-8 ferrets, it was close a couple of times but no-one died, and they're safely on the way to level 2, with enough spare money now to outfit the entire unit in chainmail. Ok disbanded the temp party. One party is time-consuming enough.
You definitely need at least 2 priests, because some high-level monsters like wraiths can take a lot of HP off weaker characters. Don't put all your eggs in one basket in other words. I have 3 priests, 3 archers and 2 warriors.
Ideally if I was starting again, I would have started with 2 warriors + 3 priests, then recruit another warrior right away, and then two archers (they train up faster and are less squishy than the others plus need money for arrows, so it's more viable to train them later). Once the priests are level 8, I'd put one priest into a garrison, and use him/her just for churning out scrolls.