5 unarmed men with basic training with crossbows and daggers could easily take out a single knight. The knight can't run them down, the knight can only fight when they engage and eventually he'll tire first. That leaves out pits, killing the horse at full gallop, ropes, maces hammers. Picks and all kinds of things that require far less skill to pull off. And a well trained knight could always be poisoned. And if it takes 4 or 5 tries to kill him you lose 20 poorly trained men. Someone loses a knight and a warhorse that will take 5 years of training and lots of treasure to replace. Even at 100 people to the knight it's a good deal in war. The knight and warhorse easily cost more than a 100 people. Knights only marched around with impunity in the middle ages because people that got caught killing them were executed. But in wars or dealing with criminals all that came off. And modern breastplates that would stop the good crossbows were called tournament breastplates that requires tackle and pulley to load. They were never used in actual warfare. But layering armor and some other advanced in armor did mitigate the crossbow at the expense of making the armor heavier, hotter and more cumbersome. Bottom line the heavier the armor the more necessary support troops where. Knights did not ride around all alone in plate like they did in Arthurian or any other fantasy knight tales.
One bolt under the arm in the knee joint etc and without support the knight was done.
To be fair most crossbows of that time were lower quality and you have to be about 30 or so feet from the knight and you'd be aiming at the gaps in the armor. The high end high powered expensive ones that took far more time to load were far more deadly. The other issue is most knights in those times wore what they could afford. Up in 1600's you'd still see poor knight wearing armour made in the 1400's because it was very expensive. Only the richest and most important wore the latest greatest armor.
Not surprising. The US army still uses Korean era equipment for some units because it's good enough for their job and it's cheap.
Eventually generals and commanders realized a well trained army with less expensive armor and gear was always the better way to go. One man covered in steel was just not cost effective.