However, there are a few exceptions: The followers in the Companions questline will automatically dismiss your current follower, and Serana will require you to dismiss them in order for her to join you on a quest (except for her first one). Most quest followers will follow you additionally to any permanent follower. There is no limit to the number of such temporary followers that you can have at one time.
As many of these NPCs are unkillable, and you can have several followers at the same time, you can use them to fight difficult enemies in a different questline. If you avoid completing the quest and go somewhere else, they may follow you around.
Some NPCs follow you temporarily for the duration of a quest. You can have at most one permanent NPC follower and one creature follower at any time. You can also find mercenaries for hire in the cities and recruit certain creatures in the game. Permanent followers usually become available after you complete a quest of some kind.
Followers can be divided into those who follow you indefinitely (until they die or are dismissed), and those who follow you temporarily (for example, during a quest). These followers can fight by your side some can also carry your equipment and perform other tasks. Various NPCs and creatures can follow you while you're playing the game and be your ally.