I've been using Serana as my sidekick and when i try and recruit another follower they always say they cant come along unless i ditch Serana. That's normal, i figured, cuz i can only have one at a time. But now that i've completed a bunch of quests for the Dark Brotherhood I am able to bring an initiate with me and she DOES follow me with Serana as well. I didnt know you could have two but it worked this time.

Does anyone know why it works with the Dark Brotherhood initiate but no one else? Not even the armoured troll can come with me unless i drop my current follower. I'm happy to have both chicks fighting alongside me, but I was wondering if anyone can explain why this is. thanks!

I can have Derkeethus, Cicero, Serana, and an Armored Frost Troll following me. (Plus my two summoned Dremoras, Derkeethus's Dremora, and Serana's undead)...

Really easy.

thanks Vaen for showing how Jusey1 completely failed to answer my question. Hopefully someone else can explain to me how it is possible to get multiple followers. I've only been able to get the two i mentioned. When i say followers, i'm not inlcuding bards and stewards and such. ONLY NPC's who follow and fight with me.
Does anyone know how i got two this time and whether or not there is a formula of some kind that gets multiple people to follow you at once? Or are there certain compatability requirements for various followers?

I didn't failed to answer your question, I choose not to answer it within that post. I had other things to attend and now I ain't as busy so I can now explain it.

It is really simple. It is a glitch. Bring any follower you want to the DBH then dismiss them... After dismissing, quickly tell them to follow you and quickly tell a DBH assassin to follow you (Before the other follower finish saying their "Let's get going!" quote) then you will end up with both followers. For me, I got Derkeethus and Cicero.

From here, you can actually go back into the Volkihar Castle Dungeons and Serana will randomly teleport to you and begin following you... There is your 3rd one.

As for a Frost Troll, you can get one just by buying one as long you don't already have an animal follower.

Your game doesn't lag with that many followers?

No.

Interestingly, I this happens if you're starting out in the Companions questline, too. In my case, I had Farkas and Serana following. Serana is truly a unique NPC in this game.