The way I was reading a forum someone was saying that a personal island could have farms and pasture and then for buildings you could do all houses and each house could have two laborers. So Max level personal island gives 5 farm/pasture and 11 buildings so 22 followers on maxed main island and if you built up the two alternate character islands i'm sure you can do the math. So any variation of effort you want to put into your alternate two characters from just farming food and mounts to refining some resources on one and crafting on the other for tools or bags.

Hard for me to really visualize since I haven't used any of it so i'm probably half regurgitating correct info.. sorry.