Followers? Like people you can travel with? As soon as they can be your companion just have them go with you and immediately dismiss them and it'll ask where you want them to go. I've got every companion and all the random encounter people at my vault 88 settlement.
If you're talking settlers, make sure your charisma is high and make sure to assign any new settlers to jobs or they'll stop showing up on their own. I think the max is in the 40's but I just have the one recruiting beacon. I've got over 20 at several locations but max most of my settlements out at 10 just because it's really hard to outfit 20+ people at every settlement.