"How many batteries do you need?"
Translation:Kiom da baterioj vi bezonas?
As this course has taught me*, bezonas (bezoni) is for needs and wish fulfillment, for lack of better terminology... Eg: I need batteries, I need to run, I have needs.
And this course has also taught me that devas (devi) is for imperative orders, like the English word, must. You must wash the car, you need to be careful (imperatively speaking), you have to help me, etc.
Basically if it's not an order but rather wish fulfillment, I've gotten by using bezoni.
*If I'm wrong about any of this, it's been a shortcoming of this particular course. These would seem to be the rules they play by.