some of these things are not 100% decided yet, but here are my answers:
1. In the beginning of the design process we considered the player having an old castle ruin or something which to rebuild over the course of the game, very similar to X-Com. Eventually we decided against that. Instead you will have one “trek” of Battle Brothers moving around the map more akin to Mount & Blade. This trek is your mobile base and can be “upgraded” in different ways. One thing will be followers who give you special bonuses/abilities on the worldmap. There will be a devblog post on that topic when we come to it .)
2. There will only be one group of Battle Brothers.
3. Just to say a number off the top of my head I’d go with 30 Battle Brothers. Maybe it’s 24, maybe 34. Brothers will get wounded in battle and take time to heal, so you always want to have enough replacements. On top of that, they tend to die very easily :) You just have to consider that all brothers will consume money and food constantly, so the bigger your roster of backup troops, the higher the maintenance.