People who do 5-6 day splits are usually working one major muscle group per workout for a total of once a week. I think youll find that if strength and power are your goals, then this is not a great split. Power relies on training your nervous system to contract your muscles in a given range of motion. So, higher frequency is required for constant reinforcement.
However, many have had success working each muscle group every 7 days when size was their goal. The reason is that it gives your muscles plenty of time to recover. These bodybuilder splits probably breakdown more muscle protein than strength programs and exhaust the muscles of their energy reserves. Arguably your muscles may be recovering in less time, so why not work them more frequently? Those who do less frequent workouts would argue that you dont lose your gain in 7 days, so its better to err on the side of more rest. This has become the norm among bodybuilders.
However, you have to ask yourself, if a person who has near perfect genetics, who devotes his entire life/day to bodybuilding, who takes the maximum amount of drugs, who has perfect nutrition, who weighs about 300 lbs with 10-12% bodyfat etc... (ie. a pro bodybuilder) does 12-20 sets per body part once a week, then what should you be doing? The answer is probably not what the pro is doing, unless youre just like him.
I personally have had better success doing lower volume6 sets, maybe 9 for short periods of time, per muscle group on a three day split with four workouts a week. That has been the best FOR ME. Less volume and I detrain. More volume and I overtrain. More frequency and I dont recover. Less frequency and I have too much time to recover. Keep in mind Im advanced, natural with good nutrition, and very good genetics. Someone else may have to do more or less. A beginner should definitely do less volume/higher frequency. I recommend you read Designing Resistence Training Programs by Fleck and Krammer.
The exception to all this, of course, is strength training. Then, I would tell you to read Zatsiorsky and many of the programs on this board, such as 5x5. And, by the way, strength training is one way to go about getting huge.