A level 5 fighter in full gear can fight and beat a dozen trained soldiers at point blank range.
Battlemaster, duelist, 18 str 14 con, plate, shield, PAM. 20 AC, +6 to hit, 2d6+1d4+18 (27.5) dpr plus maneuvers, 44 hp+10.5 second wind.
Against guards, 16 AC 11 hp 5.5 dpr at +3 to hit.
Guard has 10% hit 5% crit for .2*4.5+.15*1 --- it takes an average of 52 attacks to drop our battlemaster.
1d6+6 has a 1/3 chance to one-hit a guard, 100% chance to two-hit. Crits, or use of a BM damage die, have a 1/6 chance to not kill a guard. 1d4+6 cannot one-hit unless you use a BM die, but will kill a wounded guard.
It ends up with a bit over 1 guard killed per round (of which 0.15 is from bonus action) Action surge is about 0.85 dead guards. Give or take.
BM dice can turn .8 of a miss into a hit (worth 0.4 dead guards), grant a ripost (worth about 0.4 dead guards), boost damage (worth about 0.4 dead guards) on a non-crit or a crit (worth 0.15 dead guards because guard already dead).
Burning 1 die per action is easy.
So 2.5 guards dead round 1, followed by 1.4 guards for 2 rounds, then about 1 guard per round. After 3 rounds 5.3 guards are dead, having done about 9 damage to the fighter.
If we have N+5 guards, they last N+3 rounds against our fighter, doing N*3+9+(N+1)N/2 damage to the battlemaster.
Solving for 50 we get:
50=3N+9+N^2/2+N/2
N^2+7N+12.25=103.25
(N+3.5)^2=103.25
N+3.5=10.16
N=6.66
So 12 guards is an even fight for our level 5 fighter in mundane gear.
By level 11 the PC has a +1 weapon and shield, 16 con and 20 strength and 3 attacks. Swings are almost twice as deadly (chance to kill per swing) and the fighter gets 50% more of them. The fighter has 119.5 HP budget, a bit more than double as well. The attackers deal 1/3 less damage. And more BM dice.
Doubling number of foes makes 4x damage taken. But fighter is 3x as durable and kills almost 3x as many guards per round; so 36 soldiers at once (all in melee range!) is an even fight.
Make the foes queue up and only engage a dozen at a time (!) and the fighter can handle 100 soldiers in an even fight.
...
Is this supernatural? No mortal person could fight 100 trained soldiers like that.
And by 20 we get another massive jump in power. 4 hits, basically 100% kill and accuracy, per round. Each attacker does almost 0 damage per swing. Two action surges. More efficient uses of BM dice.
And if we optimize it gets worse.