My understanding was that the Zealots' basic stats are 2|3|0. So 3 attacks (plus charge bonus if applicable) only against Undead/Chaos.
According to the army selector and the embedded pdf, as well as jchaos97's link it should be a base of 3 attacks. Maybe there were other stats going round previously? Anyhow it should increase your winning chances in the future

Regarding the balance of forces, I tend to go with 6 Zealots brigaded with 2 bowmen and 8 Flagellants. Plus max cavalry & cannon. It is then a decision to either add some more missile units or give the Flagellants warhounds for 3 extra hits so that they can inflict more damage before they die. I usually go for the warhounds!
For me it is mainly a question about how to assemble/paint the units. I have only 4 blisters of flagellants and 1 blister of troll slayers that I can use as a proxy. I bought 8 blisters of magister militum figures in a few different poses and have 2 spare blisters of skirmishers. I can combine some of the magister militum figures with flagellant blisters to increase the number of flagellant stands.
So I can go up to 8 units of flagellants but that will make them a lot less dramatic. The remaining blisters can then be combined to get 7 units of zealots. Or I can change the balance in the zealot's favor, which might be interesting when fighting undead and chaos but probably less interesting otherwise. I think I'll settle for 7 flagellants to 8 zealots.
I can certainly see the fun in playing witch hunters. Embrace the madness right?