For this time of year usually is not the peak time for medicine ball workout. Now is usually more of a hypertrophy or strength cycle. However, that doesn't mean you shouldn't do any of them. 20 reps of anything sounds like a little bit too high just because you can't really be explosive after 19 consecutive reps. You will probably want your sets to take under 20 seconds and be explosive for every single rep. If you are doing 4 exercises I would think that 1-2 sets of 10 for each would be fine. If your season starts sometime in February or end of March, I would think your peak plyometric/power cycle would be in December or January and then back off the volume a little bit when you guys really start throwing a lot before the season.