I second using a soldering iron.....cracked LOTS of tubs using a drill, plus it gets plastic shavings everywhere and the holes aren't very clean. You can get them for less than 10 bucks.....very worth it.
As far as the amount of holes, I'd start with 3 on each side as a baseline for ventilation. Remember, the more holes you poke the more heat and humidity you are going to lose so the # of holes should be relative to the room that the tubs are placed in, if its a cold, dry room, then you only have a few holes, if its a warm, humid room, use more. Might take some tweaking to get it just right. And remember, you can always putty the holes back up in case that you accidently go overboard with them.