I would say it's over kill but it's your garden. I think you have enough for two pots. Do you have an extra nursery pot that will fit in your urn? I find nursery pots that fit inside of my pretty pots. If something gets ratty looking it get moves out of sight and a replacement goes in the pot. For a full pot I would consider pinching to get good branching. I have a yearn for urns myself. Don't forget to feed them.

For a bit of zip, you might choose something like a Red Sensation Cordyline or a spiky blue salvia to go with the petunias. Then you'd have something upright and something that trails down. If you go with just the petunias, two plants will be plenty if you use good potting mix, water regularly and feed with liquid fertilizer occasionally.

Nothing to add, great advice already, just wanted to giggle at the "yearns for urns." Funny! Is Dr. Seuss gardening now? :+)