Unless you're running a miniatures war game, you don't need mass combat rules. Determine who's going to win, and what the opposition's plan is. Fill in accordingly.
If your PCs get into a fight, grossly outnumbered, determine on what result the average attacker needs to hit each PC. Then, assume each attacker doesn't roll, but gets a number. Out of 20 attacks, the PC gets hit once for each roll result that would hit his AC.
Example: Joe Warrior has an AC 15. He gets attacked by a horde whose members have an average attack bonus of 5. So, the average member would need a 10 or higher to hit. Results 10 through 20 are 11 results out of 20...so out of 20 attacks, 11 of them hit.
So the last question is how many attacks per round does the PC face (since every 20 attacks will result in 11 hits).
-Heinrich smiles. Cynthia smiles. Hermit attacks!