NPCs who tag along on a permanent basis : not anymore - I did it previously, with anything from one to five NPCs. I never used them to fill in for things no PC could do. But having to manage NPCs who are always with the party (and, as such, interact with them frequently) detracts from me managing the rest of the world. It also increases the risk of having NPC-NPC dialogues, which are always awkward at best (unless you use socks, I guess...)
NPCs who come and go, who have a relationship with a PC but have their own life ? Almost all the time. As many as is logical according to the PCs background and the situation. Those *can* have skills and abilities the PCs don't have. Sometimes, they are significantly more powerful (and sometimes, significantly less).