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Most Jiu-Jitsu disciplines (aikido, judo, brazilian jiu-jitsu) as well as Krav Maga will teach you bare-handed counters to most weapons including knives and even handguns (if you're fast enough and the guy loses focus for a split second, you can disarm a person pointing a gun at your head rather easily). Many other disciplines also provide weapons training, but generally they assume a fair fight whether there are weapons involved or otherwise. For instance, karate as a self-defense course will incorporate barehanded takedowns and repulsion of attacks with a weapon, but on the whole it is virtually all punching and kicking and very little weapons-based offense or defense.

21 feet is forever in terms of knife attacks; your assailant must close to less than 5 feet to do damage with knife in hand. Granted, 16 feet can be covered pretty quickly. If you notice an impending attack with the assailant at 21 feet the very first thing you should do is run your butt off towards a populated area. Failing that, your forearm, despite the brachial artery, is an effective "sword"; you can use it to parry knife attacks away from yourself, close distance and body slam, grapple or punch/kick. No street fight is fair or honorable; you fight to win. It's a little known fact that kicking a girl in the groin is only slightly less effective than the same move on a guy, and it's highly recommended regardless of the circumstances. Yank hair, grab their windpipe and squeeze or try to pull it out of the neck. Use your thumbs and fingers to gouge eyes, rip off ears, bite off fingers, ears or noses, do whatever it takes to cause as much pain and incapacitation as it takes to make your assailant stop.

After a couple intense training sessions (3-4 hours a day) in a self-defense course, covering basic principles like how to throw a punch or put someone in a wrist lock, you would have the skill to aquit yourself well against any one assailant provided you are aware enough to be facing them and expecting their attack. It's then a matter of knowing your surroundings; similar to CHL carry, the knowledge of a martial art requires a heightened awareness in order to have as much lead time as possible in the case of an attack. A person in Condition Yellow should never be ambushed or jumped. And of course, the more you practice the moves correctly, the more muscle memory takes over to you can act on instinct instead of thinking about what you're doing. That makes you more able to react quickly to sudden changes, and also able to deal with multiple attackers as your body fights one person while your brain thinks about how to position yourself for the next incoming assailant.