and it depends on the caliber of your beans. I've had beans that one is 'good' and i've had beans where 'one' is "WHOOO HOOO kickass!" (I'm a pastry fan...)
I've spent good money for mediocre beans, and I've gotten lucky with cheap beans. (see above WHOO HOOO)
it depends if you want the vanilla in the front or the back of the taste. If you want it to POP, add two. If you want it to just nurture the flavor, add one.
Unless you know the quality of the beans you're getting then I'd just go with one for now.
There's always the 'next' batch...
b