Eight is the maximum limit in hand to hand because: physical exhaustion. It’s best to think of a hand to hand fight like a sprint, there’s no allowed time for recovery, you’re going at each other 100% hard and every second you waste brings you closer to losing. Unlike sparring matches or boxing rounds, there is no timer and no guarantee about what’s going to happen after. If you’re in a bad situation (and if you’re fighting you already are) then the energy you’re expending on your opponent is energy you won’t have to escape even if you do manage to win.
The battery on your body doesn’t automatically reset.
For an example, think about any sports practice or, if you don’t do sports, any gym class with a teacher who is always on your case. (Maybe has forced you to run the dreaded mile). Think about how wiped and tired you felt after a really hard practice. Now, imagine having to run for your life when you feel like you can barely stand.
Unless you’re holding out for reinforcements, time is never on your side.
Now, we created weapons to make killing easier and to limit the physical drain. The guard with a sword or a spear allows for a moment to rest and breathe while defending against an assailant, while attacking with the weapon allows for easy, fast killing blows. Try to remember that weapons weren’t invented to make combat harder, they were created to make killing quicker and less taxing on the body. Swords make it easier for a combatant to fight for a prolonged period of time. However, it is with the assumption that said combatant will be facing more than one opponent during that time.
The best way to gauge how long a fight would take in a real world format is to look at real world fencers: whether it’s ARMA, also this historical fencing, German school of Fencing, Italian/French school with foils (Olympics, best in the world in 2012), or Kendo. Don’t think about how long the matches last, but the scoring and how often the buzzers sound. Most often with the scoring, each time the buzzer sounds signals a killing blow. That is how fast the fight would last. With some of these, especially Italian School Fencing, you have fencers who have been training since they were tots and will provide a decent study (for both men and women) of what you’d get in a historical period with someone trained from a young age.
The character’s goal may not be to kill their opponent, but to debilitate them so that they cannot keep fighting. Either way, it is usually over pretty quickly.