I tend to get groups of about six followers up to 615 so I can be confident I can complete any of those that pop up, then move about six to 630, then to 645. With only three to each level, you'll get missions that you only get a small chance of completing, especially if the followers aren't epic yet. And yes, you still get plenty of XP missions for level 100 followers. I have a few toons now that only have a couple of non 100, non epic followers - makes the mission game pretty simple on those characters.