The campaign im going to be running (starts monday, will go up a few levels before we get to large battles, so i have a bit of time to get stuff) will have a lot of mass combat. Specifically, a horde invades faerun. (I posted about this a while back). Horde is split into Centuries(about 80 men or so) which are grouped into Cohorts(about 5 to 6 centuries) which are grouped in to a legion(10 cohorts), which are then put into a field army (10 legions). This makes a single army of about 50000 men. And there are 14 of these armies invading faerun at once from just north of the Silver Marches (starting with King Obould Many-Arrows). Thats approximately 700000 combatants. That does not include the 300000 other members (mostly experts and commoners) who support the armies.
Century = 80-100 men
Cohort = 400-600 men
Legion = 5000 men or so
Army = 50000 men or so
All im really needing is ways to have the mass combat, with the PCs as skirmishers/scouts on the side.