I have over 100 holes in mine. But as already mentioned, it depends on a few things. Tub size, your location(ambient temps/humidity), species of snake...etc. and also the size of your holes. my holes are an 1/8 inch. you could do less with bigger holes.

It's mostly trial and error when doing it for the first time. Start off with a few holes, see how the humidity fluctuates for a few days and add more if need be.