I think the discovery is the interesting part of the problem. Draw your space and start cramming them in. See what you get in various configurations.
Personally, I would first calculate a ridiculouly large value, just to make sure I don't get too far off. In this case, the super-upper bound might be if you smashed circles to take up all the space with none wasted. You can't do that with real circles, so this will give some guidance about the limit.
(Area of Space)/(Area of 1 Circle) = (Super-Upper Bound)