Question: I've been playing the Dawnguard dlc and I'm at the quest Toucking the Sky (Dawnguard side), and Serana is following me around in addition to my other follower (Cicero). If I complete the quest line, will she continue to follow me around or will I only have Cicero?

Answer: 
From what it looks like from [the USEP Wiki](http://www.uesp.net/wiki/Skyrim:Serana), she stops following you after the main questline. However it looks like you can enlist her to follow you again at a later point. From the wiki:

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> Her main location during gameplay depends on which faction you ally yourself with—if siding with the Dawnguard, she can be found at Fort Dawnguard; if siding with the Volkihar vampires, she can be found at Volkihar Keep. After completing the Dawnguard main questline, she can normally be found at these same two locations, although you will occasionally randomly encounter her in the wilds of Skyrim, where she can be recruited to follow you again.
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She will become a normal follower. You can part ways and enlist her again.