How many quest followers could one gather at once?
So here's what I know you can do (name followed by the quest you should start but not complete):
People
- Any normal follower (or, hey, Aranea Ienith can summon an atronach)
- Delphine + Esbern - Alduin's Wall (Esbern can summon an atronach)
- Brother Verulus - The Taste of Death
- Mercer Frey - Speaking with Silence
- Thonnir - Laid to Rest
- Avulstein Grey-Mane + Geirlund + Vidrald - Missing in Action (the latter two should join you and Avulstein at Northwatch Keep)
Animals
- Meeko (Meeko's Shack) or Stray Dog (random encounter)
- Barbas - A Daedra's Best Friend
- Horse (better yet, Shadowmere)
Summons
- Two summons (with 100 in Conjuration and the Twin Souls perk). Some dead thralls can summon atronaches, so that gives your summons one summon apiece.
which brings the total to you, nine people, two dogs, a horse, two dead thralls and four atronaches. That's 13 permanent and almost everywhere (excluding atronaches and the horse) and 18 potential. This seems pretty hypothetical and I haven't done it myself, so feel free to correct me.
Other
There's also supposedly a glitch (patch status unknown) where you can dismiss your follower, hire someone else, then immediately return to your follower and rehire him or her. I don't know if people use it with the Dark Brotherhood Initiate(s) for a reason or because it only works with them.
Also Odahviing.
In addition to what Bryan posted:
People
- Maurice Jondrelle - The Blessings of Nature
- Farkas or Vilkas - Purity
Followers who summons
- Belrand (familiar)
- Brelyna Marion (atronach)
- Eola (atronach and reanimate corpse)
Other
Apparently, the mentioned glitch for Dark Brotherhood Initiates (yes, you can get both) also works for Cicero, after The Cure for Madness. The trick is a bit different than that of mentioned by Bryan, though. Dismiss and rehire your follower. Trigger a conversation with a Dark Brotherhood Initiate or Cicero. You know it worked if you have the option to ask them to follow you.