|Running multiple companions can be a daunting task.|
That last question is up to you. You know your talents and skills as well as the needs of your game and the capabilities of your players. I can only tell you what works for me.
Firstly, know your NPC Companions. They need to feel real to you. It often helps to come up with a demeanor, coping style, and basic thought processes. What are the pillars of stability keeping them sane? What do they believe in? What are their fears, likes and dislikes? What do they think of each other? You need to get to the point where you could comfortable ad hoc the answer to any question asked of them. Often a lot of tactics used by authors when developing their characters can be used to develop your NPC Companions.
Secondly, ensure that there's plenty of character building time in-game. There needs to be a purpose to the NPCs other than simply providing 'extra skills'. The PCs could always hire a private investigator or a caravan guard in the short-term. Why keep them around?
Because they enrich the story. These are the pivotal NPCs. The ones who add heart to the tale. Whose panic makes the fear feel richer. Whose sweet aspirations remind the PC what they're fighting for. Whose humorous asides cut the tension in a way that allows it to rise greater. They may have secrets, fears, and passions of their own and, being recurring, can develop over the course of the campaign and aid the PCs in doing the same. In short, they help bring the campaign to life.
So how many can you have?
I've been in a solo game with up to eight NPC Companions on top of my own and I have to say that just like in a television series with a large ensemble cast you don't get as much time to form much of a relationship with them. If you must have such a large grouping than find out which ones your player prefers and use them whenever possible as though they were major characters in a novel (as opposed to the protagonist) and leave the rest of them as mere secondary characters.
I mean, think about the various games you've played with multiple PCs. The larger the party the less time everyone has and, more often than not, the more unequal the spotlight distribution becomes as some players consistently demand their 25% cut of the time even when there's only about 15% time available if it were split fairly (and also there are players who demand only 5% of the time regardless). Think about the kind of party size you could handle and let that be your upper maximum. Remember that you can't rely on other people to move them forward. It's all on you.
Therefore unless you think you could handle a party of eight then don't add another four NPC Companions to your cast of four PCs. If your PCs have retainers or followers then either let the players know they will be minor NPCs mostly in the background but who will nevertheless be part of the recurring cast or ask your player to play them (or see if another player will volunteer). If you can run the world plus everyone else in it, the player might well be able to run a retainer and themselves.
Personally I doubt there is generally much room for more than one NPC companions in any cast of four or more and they're generally more of the campaign enrichment variety (i.e. adopted orphans, lovers) rather than active participants in the game itself.
With smaller groups, especially solo games, you could potentially have enough NPC companions to have them bicker, flirt, and otherwise engage with each other. If you can do this (and remember to do this) often enough than marvellous. If you can't do it then I'd recommend sticking to one or two no matter how few PCs you have and then working your way up as your skill increases. It gets weird after awhile when the characters only ever talk to the PCs and never to each other.
You don't have to hold long conversations with yourself, mind. It is all about the PCs, after all. You don't even have to maintain two different characters in conversation with the PC very often. Just the occasional aside, reference to the other companion, or short conversation between two or more of them can really help with that immersion.
One other limiter to the number of NPCs you hold ... best to only have as many as you can reasonably distinguish between body language and tone of voice. It's much easier to run a conversation between Perky Voice and Dour Voice than having to go "Davenport said Blah and then Casey said Blah."
Oh, one more thing: abide by all of the rules of the NPC companion when you do so. The player should never feel upstaged by them. Their character is the protagonists. Yours come secondary. Always.
What do you guys think? Ever tried your hand at Multiple NPC Companions? Did it work out for you? Why or why not?