Returns you get for your effort.
You get the most returns for your efforts with the first 3 sets for any given body part. Regardless of how many different exercises you perform for a given bodypart the ability to generate intensity is reduced considerably by the time you finish set # 5. Considerably less and less muscle tissue will be broken down with each consecutive set there after. After that point, the curve starts to taper off but 6 sets still provides gains. After 6 sets the curve continues to slope down and you will obtain even less results for your efforts! This must be taken into consideration when trying to gain strength.
There's a strong correlation between getting the most returns for your efforts during the first 5-6 sets of an exercise and increasing your odds of developing an over-use injury when exceeding 6 sets. Your ability to generate intensity using only 1 exercise for a particular body part will be greatly diminished by the time you've finished 5-6 sets. By switching over to a second exercise after doing 5-6 sets of the first exercise, you will increase the returns you get for your efforts during the second exercise because you'll be working from a different angle. The second exercise will fully breakdown down the remaining muscle fibers that are present within the muscle group. This means trying to go past that point by inserting a 3rd exercise would be a complete waste of time because the entire muscle has already been broken down. By doing 5-6 sets a piece with your two main exercises you will break down more muscle tissue and gain more strength than if you were to perform 20 sets with 5 different exercises. Performing upwards of 16-20 sets for a body part in one session will cause you to waste a lot of time because a muscle no longer fires with optimal force once you go past doing 12 sets. Regardless of how many different exercises you use for a particular body part, the ability to generate intensity is over with after 12 work sets. This must be taken into consideration when trying to gain size.
Performing more than 12 sets for any body part will make you refrain from using great intensity on every work set. For instance, if you were on your 5th set for quads, and you knew you had 15 sets left using 3 different exercises, you wouldn't push yourself nearly as hard as if you were on your 5th set for quads, and you knew you only had 5 sets left with only 1 other exercise. After completing 12 sets for any given body part, it's virtually impossible to generate enough intensity to further break down any substantial amount of muscle tissue. By trying to do so, it creates a scenario where the Central Nervous System and joints have to work harder to keep moving the weights while the muscles are working less. You can only stimulate the muscles so much in any given training session. After reaching the point of diminishing returns, very high volume trainers who train each body part using 20 sets in one training session have to over-strain to try and make up for the muscles inability to put forth effort. This creates tons of stress and possible injuries to the muscles, tendons, and joints. When fatigue is so great that stabilizers and synergists (which generally give out faster than the prime movers) become too tired to allow maintenance of proper form, you're asking for an injury. Train till the muscles gives out; not the joints!
FACT: Adding in a few extra sets after the completion of 12 intense work sets is not going to do anything but lengthen your workout and hamper the recovery process. The muscle is sufficiently stressed and fatigued already. Lastly, the extra sets would expose you to injury through over-use!