I do not understand mass or group combat

I was reading the lord of men, (I clarify I have an idiomatic barrier and I use google for support) and I looked at the rules of mass combat, and as I understand it there is a difficulty bar, based on the situation (compromised for example) engaged 9 for example , and this has to be equal to or less than your total battle event bonus.
but I do not understand the mechanics as a whole, or the narrative on the table that I would have, although I also do not understand much about how to arm the army in the first place, I understand that for example a knight / knight bacheller, takes his mesnie (assuming the knight a poor landed) 1 serjeant, 2 infarians & archers, 2 levis (or peasants who were given a stick) all these units are groups of 6 (I think)
but I don't have much idea of how to carry this, I inquire because I am preparing a game, with a coven where many of the fellow players are noble, and I don't want to be with my pants down if I have to lead a large-scale group battle .
How could it take a battle for example in the open field between two factions (players vs npc) in the open field engaged 9 (if I assume that the players usually carry 6 grogs is the same to say ...) the same noble army landed poor vs noble landed poor , and how he would take it in a siege immient victory or defed 15.