At the beginning of every game, the DM has to answer a question for him or herself, "How do we fill the empty slots?"
The question is important because many published adventure modules require a certain minimum number of characters and frequently, there are fewer Players than required if each Player is to only have one PC each.
Now, If a Player is able to handle multiple PC's appropriately and not treat them like extensions of each other, you can easily let them run multiple PC's and get on with the game. It's good experience for becoming a DM as well.
If they can't handle multiple PC's then Hirelings and Henchmen are going to be required to fill out the spots. These are a quasi-NPC that the DM controls their thoughts and reactions but the Players handle the rest.
For me, I boil it down to this. if a PC goes looking to hire them, they are going to be a Hireling. Perhaps over time, that Hireling might become a Henchmen if things go favorably and the Players treat the Hirelings right.
If the NPC comes looking to work for the PC, they will be a Henchman from the start. This usually can happen during random encounter rolls. I have a bunch of pre-gen characters using an online character generator on hand. and I almost always have custom random encounter tables with the result of a character looking for work being one of those.
As a matter of fact, when the random encounter roll happens to turn up a person encounter, the first roll is for what type, meaning is the person looking for work, looking for love, looking for directions, etc... there are a lot of possible encounter reasons. Once it's determined that the person is looking for work, I roll on a sub-table to see if it's someone in the party specifically they come to or the party in general. The Henchman negotiation process begins.
Henchmen almost always start out with a higher favor-ability toward the PC and thus require less from them to be made happy. Hirelings on the other hand almost always suspicious and with a lower favor-ability at first so require more to gain their confidence and participation.