How many extra help can you bring around in vanilla?

In my current play, i am bringing around barbas which i dont plan to finish his quest, a random follower (vorstag), and serena which i dont plan to escord to volkihar yet, now i am wondering how many other NPC can i bring without any mod or console command? Anyone ever bring more party?

As a side note i also usually conjure undying ghost so there is that