You can get a Soldering iron from walmart for $4. You can melt as many holes as you need, but remember, you can always add more. You can't take them away.
So the best way to measure how many holes you need is to look at your humidity.
Too much humidity needs more holes. Too less humidity means you melted too many holes.
This tub set-up tutorial is really good. So take a look at it to see what we mean.
http://ball-pythons.net/forums/showt...Basics...*DUW*