Combat between large forces can be daunting to run in a roleplaying game focused on individual characters. Fortunately, game writers have produced varius mass combat systems to handle the details of figuring out who won in a fight of 1000 besiegers, three ogres, and a dragon versus a castle defended by 200 defenders, a battle wizard, and a plucky halfling rogue.
Such systems usually abstract the combat down to just a handful of dice rolls, so you don't need to make a bunch of individual attack rolls for 1200+ individual combatants.
Of course in these days of computerised gaming, why not just simulate the entire battle using the regular one-on-one combat rules? Sounds like a fun project! For someone else!
GM: Okay, we have Mark, Angus, Vader clone, two pigs, and two random Koozebanians doing the dance, with R2 and 3PO having failed their saves and joining in.
GM: Versus Chewbacca, Kermit, and Wedge doing the counter-dance.
R2-D2: This sounds like a lot of dice rolling. I've got the perfect—
GM: We'll use the mass combat system.
R2-D2: But... that's for mass combat!
GM: I've adapted it for mass dancing.
R2-D2: It reduces all of the individual skill rolls to a single opposed roll per faction!
GM: Exactly. It'll make it much easier to resolve.
GM: Is there a problem?
R2-D2: We don't get to roll all my dice!