Assuming a knight in full plate:
Well basically as long as the knight is mounted the peasants are like 1hp 4e minions. They go down like flies, with the only chance of pure luck or killing the horse. If the knight is on foot you need 20 peasants who coordinate their attacks well or it is 20 dead peasants and 1 unscarred knight.
Of course with heavy casualties involved and bigger numbers the knight gets overwhelmed at some point.
But you have to take morale into account, As soon as the knight mows down 5 or 6 of the twenty, the others run.
The peasant revolts in Germany were quite successful BUT they had professional elite mercenarys and knights who lead them and made some kind of soldiers out of them, and firearms were also in good use already , so many opponent soldiers would not be armored that heavy anymore and you had the whole allotment of pole arms already, like flails for corn improved with some long spikes, which is very effective also vs. plate armor.
A possible solution for 5e would be to treat them peasants like a swarm, saves loads of dice rolling.