The simplest and easiest way is to figure that one battery will support (roughly) one load. One load will provide between 4 to 8 draws, depending on your usage/taste preferences. Some people will be able to get more than this (2 or 3 loads) or more draws per load (up to 12), but we recommend keeping more charged batteries on hand than you think you may need.
For example, let's say you are calculating how many batteries you will need on a camping trip. A conservative estimate would be to figure that one Box filling will be sufficient for one person and will require one charged battery. Therefore, if you had two people who wanted to use the same Box twice a day for two days, that would be 8 Box loads and therefore 8 charged batteries.