How many petunias to use in my window boxes?

[BusyBee]

I was thinking of planting Night Sky petunias or trailing Surfina petunias in my garden shed's window boxes. The dimensions are 22in x 6.5in x6.5in. How many should I plant in each window box? Thanks for the help!

 

[Esther Knapicius]

what size on the plant does it say to space it at when planting?

 

[Oliver Buckle]

It is a bit of a guess, but mine recommended three to a twelve inch pot, so I would say 3 or 5. (Odd numbers always look better) per box.