There's a comment in the squat writeup that suggests it's best done with a set of 20.
Yes, yes and no.
For strength and power, sets of 5 and 3, done heavy. No joke.
But back in the day, people used their bodies as living laboratories, and someone came up with a method of gaining body weight that was so insanely effective it was the way to turn a summer vacation into a three month total body recomposition challenge. It actually rivals anabolic steroids in its effectiveness, but the reason it isn't done anymore is because it is positively brutal, painful, and most people cannot complete it.
Some nomenclature is needed, here. A "rep" is a repetition of a given exercise. Barbell over the shoulders, bend the knees until you are at least past parallel, preferably "ass to grass" - and then standing back up. A "set" is the number of repetitions you do before putting the barbell down (in most instances - the snatch, deadlift and other similar lifts have to be "reset" properly between lifts).
A "rep maximum" is the number of lifts you can do at a given weight before it is literally too heavy to continue. A "one rep maximum" is literally the heaviest weight you can lift. A three rep maximum is a weight you can lift three times, but fail no matter how hard you exert on the fourth, because you cannot handle it anymore.
To follow this routine, you need to understand that you will need rubber plates, like used in weightlifting, or a rack setup that can catch the weight without the barbell hitting the floor if you fail. NOT a Smith Machine. If you drop heavy poundage barbells on the floor which are not designed to do so, they break. Bars snap, metal plates crack.
You then need to figure out your ten rep maximum. That can be done by lifting something heavy, and figuring out where you fail, and then using Prilepin's Table to work out what your maximums at other ranges would be. It isn't effective after more than about 10, so go hard and heavy. Fail at 4, use the calculator.
And then, good friend, for the next 90 days, you will be lifting that weight three times a week. 20 times each time.
Those of us who understand English well will be scratching their heads right about. "But T.A.", I can hear you saying, "didn't you say that a 10 rep maximum means you will fail, no matter what, after the 10th time?" Correct. "So how do you lift it 20 times?"
Because you will lift that weight ten times, and only barely make it the 10th time. Then you continue to stand there with the weight over your shoulders. You do NOT set it down, lean it against something, take that stress off your body - you keep standing up with it and you support it. You stand there until you recover just enough to crouch down into a low squat, and shudderingly stand up again with nothing more than force of will, on fumes. It might take you a minute of heavy breathing and flexing the muscles to try and get more useful blood in and lactic acid out, but you take that time, with that weight over your shoulders, feeling heavier by the second. And then you do it again. Might mean a minute and a half of rest this time, but you get it. That's twelve. By the time you get to 19, you might be standing there, lungs burning, legs on fire, feeling like you are dying, legs shaking like a newborn deer trying to stand up for the first time, taking ten minutes of crippling-weight support standing to get just enough back into you to go down that one more time, standing back up on nothing more than pure testosterone, force of will, and Jedi mind power.
Then you set the weight back down as fast as you can before you collapse, throw up, or black out. It's not just possible, it's probable. Most people keep an empty bucket just by their weight rack, and make sure they take the best shit they can just before showing up to the bar.
Then you do a set of 12, three times, of the bench press, the bicep curl, the pull-up. Tax yourself, but not too much. You probably have no desire to lift anything and will have to do THAT by force of will, too. Work every muscle you can.
Then, you lie down. You take it easy. You will not be able to walk for a while. You will probably not walk right for two days. You will probably seek out the handicapped stall at the public toilet the next day because you literally cannot sit down normally, but can only fall onto a seat of any kind. Forget any other kind of training. You won't be running, playing spccer, doing landscaping, roofing or any other hard endeavor. In fact, it is recommended you nap and get at least ten hours' sleep if you can.
Because in two days time, you do it all over again.
And at the end of the week, you add five pounds to the bar for the next week's total. Each week. For 12 weeks.
See now why few people ever complete this? But those who do, and get the right nutrition and sleep and rehab, find out that they're not only a hell of a lot stronger in every respect, they're also having to buy new clothes, because they no longer fit into anything they own. A gain of twenty pounds of muscle is not unheard of, and that's a remarkable achievement.
They also walk away with Navy SEAL-level of mind control. When everything is telling you "I can't..... just quit!" and you don't, you learn you are more than capable of doing things that are superhuman.
Meanwhile, your body is responding to body signals of "oh my God, I almost died doing this" and "oh my God, I need to do it again" by very rapidly getting into gear and taking every availble ounce of protein and calories and turning that into tissue repair, and more muscle fibers. Evolution is one of those things where homeostasis is nice, but if the organism needs to do something to live, it damn well does it.
Paired with this is the infamous "get big drink", which busted family food budgets and had gastric distress added to your woes. There are various variants of it, but the main components were a gallon of whole milk, plus an entire brick of ice cream, a cup of black strap molasses, wheat germ, and a ton of other nutrient and calorie dense foods. Normally drinking that daily, along with meals, would be a recipe for becoming fat, but in this instance it gave you the calories and proteins and fats to carry on. Many boys carried through with the routine, fighting through the "gallon of milk a day" diarrhea knowing that if they didn't, all those calories would turn them into large blobs of lard.
You didn't do this forever - you tried for 90 days, and even getting to 80 or so was an enormous achievement. You took it EASY for a while thereafter, but after a month or two of normal weight training, you could go for it again.
With the advent of streroids, where all you had to do was buy dodgy drugs that could kill you - well, that was far preferable to the amount of suffering that a routine like this put on you. It's not really in vogue anymore, but if you want to gain 30lb of muscle as fast as possible, hey, it's there. You just better be up to it.