[Al Skierkiewicz]

merybar,
In reality there is no way to really answer this. Some teams eat a battery almost dead in one match while others can easily go two or three matches or longer. If you plan to just sit and watch during the finals, then you won't need too many. However, if you plan to make the finals, (You have planned to that haven't you?) then you will need a few extra for the later matches. Although you could go begging, you might want to have a few of your own just in case. If you are a small bot with omni wheels and tank drive, then you might be able to get by with three or four. If you don't use omni's and you have a lot of functionality and draw lots of current in turns (this is a turning year ya know!) then I would go with some of the recommendations above.