The number of kettlebells you need depends on your goals. Most of the time, kettlebell exercises use only one kettlebell at a time. If you are just getting into kettlebell training, we recommend two kettlebells to start with, one lighter and one heavier, so you can get a feel for the different types of training you’ll do.
The lighter one can be used for technique training for new kettlebell movements, such as kettlebell swings and stability work, and the heavier kettlebell can be used for exercises you’d commonly do with a dumbbell, such as rows and squats.
If you decide to get more kettlebells, you’ll probably only need one of each weight that you plan on using e.g., one 8kg, one 12kg, one 16 kg, one 20kg etc. But start with two and see whether kettlebells are for you.
You could even just buy one adjustable Kettlebell Handle, such as the Ironmaster Adjustable Kettlebell, so you can add weights to the one handle, saving space in your home gym or studio!