Squat jumps should be done in sets, maintaining speed of movement and form. Repetitions should start at a number that is easy to remember like five or ten and always increase when it becomes easy. Stop if you get to rep eight or nine and you’re not even getting off the floor anymore.
Again, remember the overload principle. Once you get to that point that you’re not leaving the floor, you have accomplished overload. Going further isn’t worth the effort. Stop, rest for 30 seconds or so, and then go again. You may not be able to get to the same number of reps in set two or three but you will overload the muscles if done properly.