The absolute easiest way to do this is to unlock the ability to buy new followers from your camp, then enter a dungeon and use the ability to teleport back to camp, ending the run. Once there is another follower to purchase repeat as necessary. Followers can also give you side missions that give you sick followers or old followers as well, which count. This is a story achievement as 20 followers are needed to get to the final area. It should be noted that the final area needed you to have 20 followers every time you attempted it, so going below that number means you will need to get more. Buying a follower from camp also means that no follower can be bought in that dungeon run, although they can be rescued or found within the events of that run.