What body factors limit the number of punches in a combo?

Question

What limits the number of quick punches a boxer is able to throw in a combo (eg. a 1-2-1-2... or 1-2-3-2... combo)? Is it related to breathing and cardiovascular ability, muscular strength, weight or something else?

I'm not necessarily referring to a combat situation, where other factors (the opponent for example) will have an influence. Even while punching a bag or pads, or even shadowboxing, some people seem comfortable throwing 15-20 punch combos, while others struggle with more than 4.

---

EDIT : (1) The punches in question are full-effort, with proper form, how they would (should?) be in an actual fight.

(2) '15-20 punch combos' may be misleading, I actually mean a string of combos without 're-setting'. A simple example being 1-2-1-2-1-2-1-2-1-2-1-2-1-2-1-2-1-2-1-2, (1-jab, 2-cross) which is the ending of this workout.

---

While experience, conditioning, practice etc. will surely play a role, that is not what I'm looking for here; I'm trying to understand what specific body factors make one stop (and therefore, need to be worked upon).

Background: Boxing novice, no real-life combat experience, not overweight, distance-runner.

Answer 1

Even someone with very basic ability should be able to alternate left and right punch punches indefinitely at some speed and intensity level, and they could trivially add some variety and footwork and call it a combination. Still, if you're just bouncing off the target rather than slamming into it, it doesn't mean much. What varies considerably though is the speed and intensity the proponent can manage; factors affecting that include balance, stance, footwork, hip and shoulder rotation, and - generally - the punching technique itself.

At one extreme, if your back foot points out too much to the side, you're very side on, and your feet are almost on a line to the target, you'll find it hard to throw a decent cross and will tend to lose balance, and if you throw weight behind your jab it will tend to lunge into the target and be slow and clumsy to recover from. After a few "effort-full" punches you'll feel like you need time to "reset" and recover your position. Even someone who's quite successful and superficially looks formidable may exhibit this type of problem - e.g. here's a youtube video where the back foot doesn't rotate well and the back leg isn't adding much power to the hit - consequently the back hand isn't driven through the target powerfully (see "power shots" from 3:12): rather, it's thrown with an expectation that it won't move the bag and pulled short without the back hip ever rotating to the front.

That contrasts with a good stance where your body and back foot are rotated perhaps 45 degrees from a line to your target, and when you go to cross there's plenty of room for your hips to turn (your back knee should rotate downwards and forwards rather than staying pointing out sideways), so the shoulders can square up during the cross. If your stance is just the right width and depth and you're turning the right way at the right time, you'll find it possible to use both hands and throw hard and fast combinations. He's not doing much, but look carefully at the direction faced by Tyson's shoulders, hips, knees and feet as he just moves around the bag in this youtube clip. He's balanced and ready to mix it up with both hands with real power.

With good technique, you can develop a feel for how your movement will be resisted by the target so you can factor that in to keep balanced and well positioned at the end of each individual technique, which allows you to chain them freely.