Welcome to GoY Paul. Depends on whether you are going to put other plants with them . . . e.g. Bacopa, Fuchsia, etc. Normally, a total of 6 plants around the edges and one in the centre will fill the basket nicely once they bush out.

It also depends on whether you're using ivy leaved trailing geraniums, or cascade geraniums, but if its the former, and you don't want anything else in there, I'd say 4 o 5.