Answer: If the circle is 10 feet across (5 foot radius) the "length" around the bed is about 31 feet. Pentas will cover about 3-4 feet, so it should take 8-10 plants for the bed.
I don't think you can carry a penta through the winter in your location, even with protection (now you've gotta try it, right?) so I'd count on replanting each year.
Good luck with your plants.
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