There's no real right answer for that, it all depends on the conditions of your unique setup. I'd start with about 6 on either side, and see how it goes.

Remember, the less holes you punch, the more heat and humidity the tub will retain. I solder holes through on the back 3 sides of the tub, leaving the front of the tub facing out the rack totally clean. Seems to help the cool side maintain a decent temperate and holds humidity while keeping the warm side ventilated enought to prevent a lot of heat buildup.