How many holes in a tub is too few?

I'm setting up a tub for my first snake and I'm wondering if I should be worried about my snake not getting enough air? It is a 30 quart tub and I put around 3 pencil sized holes on each side, including the top, is this okay? I live in a drier climate so I figure putting fewer holes will let me maintain my humidity a bit more.

Not sure what the minimum is, but I have about 12 holes on each long side and 6 on each short side of my tub, which are each roughly the diameter of a pencil. If you use a humidity retaining substrate like cypress mulch or coconut husk there should be plenty of humidity. Using a tub instead of a screen top glass tank already makes a big difference. I try to maximize airflow for the snake without sacrificing humidity.

I added a few more holes, now 6 on each long side. I feel like I shouldn't need quite so many considering this is a species that spends most of its time in burrows. Plus i don't have those substrates available to me so i will be using paper towel for now..