Generally speaking, repeat 8- 12 times each time, and the weight of dumbbells should make you feel challenged. This weight is the most effective for increasing muscle and strength.
Considering that you are 16 years old, your height is 172cm and your weight is 50kg, you can choose a lighter dumbbell to start practicing, such as 10-20kg dumbbell. With the increase of strength, the weight of dumbbells can be gradually increased.
At the same time, we should pay attention to different types of dumbbells (such as dumbbells with adjustable weight and dumbbells with fixed weight) and different training objectives (such as muscle gain, strength improvement or endurance training). It may be necessary to adjust the weight of dumbbells according to personal needs.
Finally, no matter what weight dumbbell you choose, the correct posture and the correct training method are very important. If you feel uncomfortable or painful during exercise, you should stop immediately and seek professional fitness advice.