It really depends on the petunia you're going to use. But it's a simple math problem, really. First, you need to find how many square feet are in your bed. For the part that is a rectangle, multiply the length of that part times the width in FEET. This will give you the square footage of that part. Then measure the smaller area, trying to mark it off into rectangle/s or square/s. Do the same math for each area. If you end up with a triangle, pretend it's a rectangle or square and take half of that. Now add all the different areas (square footages) together.

Now, look at the tag on your petunias. If they say to plant them three feet apart, that means that each plant will fill in three square feet. If they say plant them one foot apart, each plant will cover one square foot. Get it? Divide the total square footage of your bed (that's the area), by the square footage each plant will cover and you know how many plants you will need.

And you thought you'd never use that geometry class you took in l0th grade......:)

(It's a really good idea to put this information somewhere safe so you don't have to do all this measuring every year.)

To fill an area I plant petunias 10 inches apart.
And as for the number of flats....that depends on how many plants per flat...
I say start with 4 flats and get more if you need to....but do the math....my guess of 4 flats is without any calculations, just top of my head guess and experience.
Linda C