Where to place vents?

I have a 4x2x1 PVC enclosure that I need to install vents into. I'm planning on probably buying some circular vents a couple of inches wide. My question is, where do I place them and how many do I need? Any help would be appreciated!

What snake are you planning on putting into the enclosure? If you need a high humidity area you're going to want less vents and you would probably want them high. And visa versa for a low humidity enclosure.