I was going through the vampire questline and felt I needed a break from it, so I went down to Whiterun to start the Companions questline. I kept Serana on as a follower. About the time where you're supposed to go with Farkas to reclaim a piece of Wuuthrad, I was fully expecting Serana to part company and leave me with Farkas. I ended up going there and delving into the crypt with both Serana and Farkas.

So here you have two vampires and a werewolf going up against draugr and Silver Hand. Just unexpected... and wild! I didn't think you could have two followers?

Of course when that quest was over Farkas left the trio, so now I have Janassa and Serana both following me.

if you go on to the UESP sky rim followers page you will find you can have up to 5-6 followers if you plan correctly, plus more through quest lines

I think 2 thralls + 1 housecarl/mercenary is enough to take over nearly anything.

I got Marcurio and Serana. We make a cute little trio of mages.

yes, that seems to be the case. I mentioned it somewhere a good while ago... the catch is, you can't send her to be cured. Once you do, bye-bye second follower. That kinda svcks for console users.

I remember first time I went through the Thieves Guild quest line and was in the Nightingale part. It was me, my follower Lydia and the two other Nightingales. Though Lydia got kicked by the game eventually but rejoined me when I left the caves.

But thats why I like playing on the PC now. with a mod I can take as many as I like.

You + Follower + 2 Dead Thralls + 3 Atronachs (if you give each of them an Atronach Staff, they will summon) + Dog + Horse = a company of 9.

Until you start the mission Prophet, serana is not counted as a real follower. As she also won't take any of the stuff you want to trade. As soon as you start Prophet she will not join you if you have a follower.