I never had any information myself about how many followers you can have. But the other days I saw a youtube video where someone was moving around and around 3 to 4 other people where following him.. dont know if this people were followers or part of a quest where you work with a group of npcs..
Bethesda designed the game so that you can have only 1 follower and 1 dog with you at anytime.
The exception to that is a quest follower (ie if you already have a follower on an active quest, then you can also have the 1 follower and 1 dog).
All the follower mods out right now either force the second follower into the dog slot or unlimited followers, but have broken follower functionality.
Having full-functionality multiple followers will be important, IMO, especially when increased spawn mods come out and you absolutely need more than, say, Lydia and Meeko the dog for help.