Basically, with ThemeForest licensing, one standard license = one website. Whether or not you’re using a Child theme really doesn’t make any difference, as you’re still obviously using the Parent theme for each project.
Here’s the outline on licensing from ThemeForest: http://themeforest.net/licenses/
So, you’d need to purchase a standard license for each website. Or, if you were to purchase an extended license you would not have any restrictions in this regard. I suppose the determining factor would just be getting a rough estimation of how many times you’re planning to actually use the theme and seeing if an extended license would actually be worth the price. I would say for most people, it’s pretty rare that someone would be using the theme enough to warrant purchasing an extended license, opposed to several standard licenses.