I have several 32 qt SweaterBox racks from Rich at RBI. For the 32 qt Iris tubs, I use 7 x 1/4 inch holes low on the front of the tub, and 7 x 1/4 inch holes high on the back of the tub (promotes air exchange). You really don't need any on the sides unless your humidty runs too high.

I would advise you start with less holes than I use, and adjust from there based on your ambient environment.