I tend to think a 100 peasants would kill 2 knights, no problem, 50 peasants, well organized, could also do the job. However less than that, say 20-30, and its more based upon how much training these peasants have, skill with weapons and fanaticism.
For 10-20 units to defeat 2 knights, they would simply have to be BETTER than peasants imo.
That being said, while high, high quality troops = one unit per soldier ..... really, really crappy troops can have one man = 50 peasants, skeletons, goblins ... although any unit representation over unit actuality kind of bleeds into unrealism AKA Kings Bounty or HOMM .... which is just a really, really odd way of fighting battles, imo
Perhaps the answer is bonus modifiers in natural terrain, or useful habitat. Lowly troops could use this, in order to Zerg to defend their own lair ... kind of like elves in their home-woods terrain. However, alternate terrains which are drastically different (plains or desert for elves, desert/jungles/mountains for normal peasants, would have penalty modifiers.
Now, elite units should probably be immune to such penalties and bonuses, just to extrapolate that peasants CAN be good under, as you have said, the right circumstances.