Caravans only exist because going alone is dangerous. Due to the pressures of economics and danger, every person in a caravan is a liability, because they require carrying food, water, and personal effects that take up valuable room that could otherwise be filled with trade goods. Anyone who is an unnecessary liability is a threat to the success (i.e., survival) of the caravan. This means that nobody in a caravan is going to be there without having a very good reason.
There are two good reasons for someone to be in a caravan:
- They are an employee whose cost (in terms of space and/or pay) is justified by improving the chance of the caravan's survival and profitability
- They're paying to tag along with the caravan, for safety in numbers, and paying well enough to offset the fact/risk that they're an "unknown quantity"
This means that, in terms of NPC types, you have a few well-defined caravan employee types, but also room for every other character type you can think of (so long as they have a reason to travel).
Legit caravan jobs
There are very few jobs that need doing in a caravan.
- You need drovers: the people who know how to control the creatures, keep them moving at a pace both sustainable for beast-survival reasons and fast enough for economic reasons.
- You need people who can tend the animals: grooming, feeding, watering, bedding them down, and perhaps doctoring them. These may be the same as the drivers, but because this is an end-of-day job and the drivers will be tired, they may be separate jobs or jobs with overlap (say, done by the apprentice or backup drivers).
- You have the trader/merchant/owner/trader. A small caravan may have one master, while larger caravans may be multiple joined together for protection, or if one large caravan owned by the same people/person, there may be multiple merchants simply because they need that many negotiators at the destination. These may also be drivers, but if they're wealthy enough, they may also choose to not dirty their hands with that work. Traders may be owners, or they may be employees of an owner back home.
- Guards. Caravans primarily exist to make it easier for more stuff to be guarded by fewer people. These may also overlap with drivers, if drivers rotate through shifts between droving and guarding.
There you have it: four character types that are integral to a caravan, all but one which are optional if people's jobs overlap. Jobs other than these are dead weight, and simply won't be part of the official caravan.
Among paying travellers, there are no generalisations, so you can meet any sort of person. All you need is for someone to have a motive to go from point A to distance point B, and they'll have a reason to join up with a caravan.
All of these travellers, to benefit from the protection offered by the caravan, will have to carry their weight. They'll either pay for the privilege in hard coin or other economic trade, or they'll work for their passage. If the latter, that means that they will actually be one of the NPC types above (just like your PCs will be guards), except that they might have an extra background layer beyond simply being an employee of the caravan.
This category lets you add a few interesting NPCs for variety, but due to the nature of travellers covering the whole spectrum of people, there's no way to give guidance to narrow down your options.
Are not possible to generalise about.
Caravans are economic enterprises. Think of them as one-time, one-purpose temporary companies formed for a specific job. Now think about how much variety there is in how modern companies are crewed, in terms of numbers, job separation or overlap, and specific mixes of skills. Caravans are the same: they will have exactly the jobs and numbers that the caravan master believes will give them the best chance of success, and that means that a caravan could be anywhere from tiny and overstaffed, to huge and understaffed, to huge and overstaffed and tiny and understaffed.
How rich is the owner? How much risk are they comfortable with? Are they risking their lives as part of the caravan, or just risking some pocket money and staying home? You can literally have any numbers you like; and on top of that, caravan masters can make poor choices in caravan composition, just like people can make poor choices in building modern companies — so there is no limit to how overspent or understaffed a caravan can be. Lots of skilled drovers, and tenders, but skimping on the guards and buying unhealthy beasts? Sure, that could happen. Overworking the drivers by having them drive all day and be tenders at night? Sure, could happen.
The numbers are ultimately up to you. This is a game too, and you don't have to be super-correct in these details. The take-home message you should get from all these unknowable variables is that you shouldn't be picking numbers and NPCs based on what's "right", you should be picking them based on what will seem plausible to your players, and be making choices that give you the NPCs you want to play with during the trip.