So I usually see weight sets done in groups of 8 or 10. Basically the advice is to pick the heaviest weight that you can lift for the set, making sure the last of the set is "just barely". Just how much does the number matter? If I can do, say, 25 pound curl 8 times, why not say 35 6 times, or 20 12 times?
What's the greater principal?
Allow me to quote the following:
A training study from the Stuart Phillips-led research team at McMaster University in Canada confirmed their earlier finding that light weights are just as effective for building muscle as heavy weights; the key to building muscle is the effort put into each set. In an important new finding, their data hewed to the long-established specificity principle in building strength; to lift heavy weights you must lift heavy weights. Finally, their results also suggested that additional volume—more sets—may build more muscle.
For more information on the study, here is the link:
Study Confirms Light & Heavy Weights Build Muscle, Specificity principle in Building Strength
It all depends on what your personal fitness goals are. You can vary your training parameters (rest time, intensity, volume, exercise selection) to help you achieve them. As Rich said, you should read Practical Programming. It will help you to learn how to create your own training programs.
In matters of style, swim with the current. In matters of principle, stand like a rock.
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