Put this in a file called pre-commit in .git/hooks/

#!/bin/sh
# To enable this hook, rename this file to "pre-commit".

git rev-list --count master > build_number
git add build_number

Now there is a file called build_number in the root of your directory that counts the number of commits on your master branch (which is your build number).

Make sure the permissions on the file you created are the same as the samples in that directory.