The Earthboxes are very specific about how many plants to put into each. http://www.earthbox.com/consumer/grow.html
2 tomato plants, 4 cucumbers, 6 strawberry plants is all you want to stuff into each box. I heeded advise and resisted the urge to put more then the recommended amount of plants into each box
Earthbox forum is a superb place to ask questions about this: http://forum.earthbox.com/index.php?board=4.0