I've always kept this idea in mind, since starting to train with serious intent years ago. Three reps will do for you what 20 reps will not and vice versa. 20 reps can do what 3 reps can not. 6X3's can build size as well as 5X20's reps can for others. And both these rep ranges can build strength. TUT (time under tension) is not always gained by a higher rep range of 20 or so. It can be accomplished very well with a moderate range of 6 to 8 reps, when the weight used is slower and controlled, in both the positive and negative movement. (one rep only of 30-60 seconds, if you follow the Art Jones method...example..the pull-up or dip...a true killer that most men can not accomplish or would not care to) That is the whole idea, TIME under tension. As far as a workout goes, it's not the number of reps in one set for any muscle but the total number of reps in the entire workout for that muscle group.
No offense to any one but most of the type I or II muscle fiber protocol really does not apply to everyone. Fast twitch or slow twitch training does not cover the whole scope of lifting a weight. Weither the goal is muscular development or strength increase. A planned leg building focus for 6 to 12 month should be considered. Too many guys just think of 3 or 4 weeks down the line when trying to work a certain muscle group (example: they want to get ready for summer...instant muscles). Training is a long haul undertaking, not a quick fix.
So it might be advised to mix up the rep scheme from 4 to 6-8 weeks, of say 5X5's (excellent muscle mass builder for thousands of men over the years). Than change to 8 to 12 reps too (moderate weight with attention to TUT) the next 4-6 week cycle. The final cycle being 15 to 20 reps. After that, repeat the 5X5 cycle again. May want to take at least a week away from training after each cycle ended. The idea is not to become too stagnant with the rep range, if a full potential for results is desired. This is not a true periodization system for BB'ing but somewhat similar. The exercises done and set's selected will be a personal choice here. Keep in mind that, for the most part, less is better when it comes to set's per muscle group.
I assume most guy's here have a working knowledge of diet, meeting the daily quality calorie (fats, oils, protein & carbs) requirements and more to add pure muscle mass and gain strength. Enough said about that than. Good Luck.