Understanding how many sets and reps you should do is crucial for an effective programme. Under or over training will likely result in a lack of adaptation and/or injury. However, the research has been inconclusive due to the high amount of variables that come into play (training age, training goal etc). However, there are a few basic principles that are generally agreed upon that I will go through.
The amount of sets and reps you need to do will of course depend on your subjective goal. Even if your overall goal is "A", each meso cycle will have its own goal. For example, you could be a marathon runner with the goal of improving your London Marathon time. However, in the months running up to the event you may need to put more muscle tissue on. This will mean you will have to go through a hypertrophy phase to put on muscle mass (which in turn will allow for greater advancements in strength). So in the context of this example, although the overall goal is endurance, the acute focus is hypertrophy - therefore the reps and sets (and potentially exercise selection) needs to reflect the focus of the meso cycle.
Leading on from the previous paragraph - it is clear that the sets & reps (volume) will need to change depending on the goal. Although it is not clear cut (just because you are in a hypertrophy phase it doesn't mean you won't stimulate strength adaptation) varying volumes will bring about different adaptations.
Above is a chart called the "Prilepins Chart". It is a very useful chart to gauge what kind of volume you will need to do to achieve your designed adaptation. Using the chart, if you wanted to work on your Strength then you'd need to work with weight more than 80% of your 1 rep max for 5-10 reps for 4-8 sets. Of course this is just a guide as volume can be manipulated depending on chronological age, training age, goal, phase, where they are in relation to competition and fatigue levels.
Research has concluded that the higher the weight, the less overall volume is required and visa versa with the lower weights.
It is crucial to understand what 1rm you are basing your lifts on. For example, if my Squat was 200kg and my Snatch was 120kg - basing my Snatch work off my Squat would be a very silly thing to do. Similarly it would be silly to base my Squat work off my Snatch as the weights would be far too low to bring about any adaptation.
I hope this helps with understanding what kind of weights, reps and sets you need to do to bring about adaptation! If you have any questions on this topic then please feel free to get in touch with me!