How many leaders for the long Campaign... hmmm well I'm almost on the 3rd Leader for the short campaign I'm about to finish. I started slow but now I'm totally Blitzing them so I think that 3 Faction Leaders for the short is about average.

The long campaign though will take much longer - maybe 2 to 3 times longer as you need to hold Rome and have 50 Provinces or something. Both of those are really daunting and will probably take upwards of 7 to 10 Faction Leaders to do it.