On my 28qt sterilites I put as many holes as I can fit down each side 1" apart..comes out to about 18 or 19 I think. I use a soldering iron to melt my holes, but have used a drill b4 and when I did I used a 1/4" drill bit. Also I keep my reptile room at 76-78*F and around 35-45% humidity so you might not need as many holes as that.