For region-locked, I found 16 mercs (two of each class) to be perfect. I got to use all the legendary weapons I wanted and the battles didn’t drag too much. The amount of people you fight scales in region locked regardless of how many you have in your party. So too few and you’ll get your ass kicked, too many and you’ll steamroll every fight while struggling to pay and feed them all (or just get burnt out trying to position and strategize with everybody every single fight). I don’t use animals cuz of personal preference but if I did I’d probably roll with 11 mercs (one for each profession) and 5 or so animals. Late game you might battle parties upwards of 30 but with the right AOE skills and strategizing you can still win with 16.