I did my 16' Scamp
and I would suggest that 13' yards would be the very min.
How much you need has a lot to do with how you plan on finishing the cushions. Box sides will take more than a rollover side. If you are making your own edging trim you will need extra fabric for that. I see some folks showing a pattern layout that isnt really in best sewing practises but it will result in less fabric being used though. Best practises is to layout your pattern all running in the same direction on the fabric weave. If you have a repeating pattern for your fabric, you will need to match the pattern from one section to the next, so layout placement will eat up a bit more fabric.