I don't know how many the elite teams have, but my mentor once told me, "You can never have enough batteries". Now, while that's a bit of an exaggeration, I suggest having around 8-10 with you. We usually have 1 on the robot, at least 3 at the ready, 1 in our robot cart, and at least 3 charging. Using this method, we've never run into a situation where we were lost on batteries. It's a good idea, though, to get around 80% of your batteries charged and at the ready near your final rounds, since you won't have enough time to go back to your pits and change them.