My idea for escaping prisoners and deserting companions is one that kinda changes the whole dynamic of the prisoner and leadership skills.
First of all, imagine that at the beginning, you could hire as many people as you want. (with restrictions on population of the area, money, supplies and such)
You may have 0 leadership and a massive army of 100, but because you don't have any leadership skills, you'll see your army deserting in the dozens if your morale goes down even in the slightest bit.
So if you start without any leadership, and hire a huge number of guys, they'll all bugger off as soon as you lose a battle (or a large number of men in a battle) or fail to keep them paid or fed.
Even with a leadership skill of 10, you'll still experience desertions seemingly at random. With leadership of 10, you might be able to manage a group of 100 without any desertions (or maybe the odd one), but once you increase that size to 200, the rate of desertions increases, until you find that at a huge number of troops, you'll lose many of them during the course of a single day.
Basically, the chance of a certain person deserting should be directly proportional to the number of people present in your party. Those of you who enjoy mathematics would recognise this relationship as a simple differential equation, whose form you often see in population numbers.
So what did that mean? Basically, we take our level 10 leadership guy again. He's just hired 300 soldiers. In the first day, he loses ****loads, simply because he can't control them all. In the second day, he loses a large amount, but not as much as before. He now has 200 men. Over the course of the next week he loses 100 of these 200 men, leaving him with only 100. But once he's down to these 100 loyal men, he experiences hardly any desertions.
This leadership could also take into account other factors. Like a knight would be more likely to desert than a pikeman, simply because he has a larger ego, and probably doesn't like the idea of being commanded by an equal. That way, one wouldn't be able to sustain a large army of pure knights. You'd either have a large army of peasants, or a small band of knights.
This soldier rank specific chance to desert could be dependant on your own level (or rank within the army). Everything else being equal, a knight is more likely to desert your army if you're level 1, than he is if you're level 23.
Ok. Now that i've explained the leadership thing in huge detail, i'll explain the prisoner thing using the same ideas.
Basically, you can hold an unlimited number of prisoners. (the only limit being the number you capture in battle)
However, unless your prisoner management is really high, you'll find that most of them manage to escape.
The chance of prisoner escaping could be dependant on a couple of things.
1. your prisoner management skill. The more you have, the higher the limit of prisoners you can have before they start escaping in large numbers.
2. Party size. More people means more people to watch over your prisoners. (hay, manhunters and slave drivers and such could count as 1.5 or 2 men for this situation) It doesn't make sense that one person, even with all the prisoner management skill in the world, can keep control of 20 prisoners all by himself.
