The choice of weight is generally to choose 60% of the maximum weight you can lift at a time as the training weight. For example, one-arm flexion-extension dumbbell can lift 20KG at most at a time, so your training weight should be 12KG. The requirement is that when you complete 8~ 12 in a group, you should feel tired rather than exhausted. When your muscle strength increases, you can easily finish 12, then you should increase your weight and make your muscles feel tired when you finish.