As noted, there are companion NPC rules in DMG, which can be used to accommodate for the occasional assistant.
But the focal point of the story is supposed to be the PC's, so long term NPC's are not recommended. There is nothing wrong with having strongholds, NPC's who serve the PC's in these strongholds, allies and minions who work for the PC's etc. But these are all story elements and don't necessarily need stats. The only creatures that need stats are those who will be involved in combat encounters.
As such, the squire who takes care of the horses, and the apprentice who keeps the wizard's tea warm can certainly be around. They just don't need stats.
Having said that, involving NPC allies in combat can bring a new dimension to the game. In my current campaign, the players are assisting some elves with the defense of their grove against some gnolls. I've statted out some tree ents as companion NPC's and gave them to the players to control for a couple encounters. They had a blast. I also statted out a unit of elven archers (one huge swarm), who helped out in a couple encounters. This gives the sense that there is a mass combat going on, while keeping the focus on the PC's, since the activation of a dozen elves goes by as quickly as one creature. It also added an extra element that the PC's wanted to protect the elf archers from harm as best they could so it gave them a secondary objective, while the gnolls tried to break through the PC line to get to the elves.
The squire, the apprentice, the cook, etc. can also have events happen "off camera" that could be summed up later. For instance an area is plagued by goblins, the heroes break camp to go find their lair, leaving their entourage safely hidden in a copse of trees. When they come back, there is a half dozen dead goblins, looks like the squire's now broken sword is embedded in one (and he's holding an injured wrist), the apprentice seems to have burned a goblin's face off with hot tea, and the cook took out the other 4 with a thrown skewer, a meat cleaver to the head, a fork to the chest, and a frying pan to the groin (you think that last one is still moving around a bit, groaning).
I think "on camera" NPC's should be used sparingly, and to add color or purpose to a fight more so than to give the PC's some benefit. "Off camera" you can go wild.