How many hardy geraniums or petunias to a 30x70cm window box?

When you say geraniums, I take it you mean pelargoniums, not hardy Geranium - 5 on their own, at a push, assuming they're upright ones, or alternatively, 3, with something like 4 million bell petunias trailing over the edges, one at either end and two either side of the central geranium. I've called them Million Bell Petunias, but they're now known as Calibrachoa. Probably about 5 ordinary non trailing petunias if they're on their own.