Stretching does not improve your ability to jump high. That means that being able to move your joints beyond the normal range of motion can not help your ability.
It is true you need to be able to move your joints within the range of motion for any activity, but being more flexible can not improve your performance.
Research has proved that per event stretching does not improve performance or reduce injury.
Stretching after training is more important.
I don't really stretch a lot before/after I jump. I have a 42' in.or so vertical. The best tip I can give is this:
-Don't do the jumping crap people teach you on free programs.
-Do squats (With or without weights)
-Perform lunges
-Do step ups (Both feet)
-Stretch the following: Quadriceps, Hamstrings, and Calves.
As for the reps, just do as many as you can without straining yourself or feeling weak.
do air alert best workout that makes you jump wayyyyy higher
jump rope.