I think I already answered in IM but just in case,

Dismiss any existing follower you have
If that doesn't work,
type in the console setplayerfollowercount to 0
If that doesn't work,
do this: https://support.steampowered.com/kb_article.php?ref=2037-QEUH-3335

And if you want multiple followers, download Ultimate Follower Overhaul here, make sure to read the instructions for installing and setting it up because it's a bit more complicated than most mods and can get bugged otherwise
http://www.nexusmods.com/skyrim/mods/14037/?

the UFO still doesn't help me.

Did you try the console command? ~ to open it, then set playerfollowercount to 0. That should clear any existing follower no matter what. If UFO still won't work you could try this one, someone said it worked with this mod, I haven't tested it myself though.

http://www.nexusmods.com/skyrim/mods/31581/?

And there are other multiple follower mods, I just stopped testing new ones since UFO worked for me.

okay thanks

And if i type setplayerfollowercount4, will it allow me to use 4 followers?

And i tried that mod too, still didn't work for me.

no, setplayerfollowercount is a flag applied to your character that tells followers how to react, 0 means you have no followers and 1 means you have a follower and no others will join. It should change automatically when you get a follower but sometimes it gets bugged and does not. Setting it to 0 automatically dismisses any existing follower you have. Only other thing I can think of is if you have an active forced follower quest, like when Aela joins you after you become a werewolf to rescue that guy, you'd have to finish or drop that quest first. Other than that, don't know what else to do then if it still isn't working, sorry.