Hi Marion the answer to this is, I don't know. But here is what I would do.
I would take a piece of paper and draw the outline of the deck area. Then draw on the planking arrangements at the width of plank used. Including the but joints at the ends of the planks, unless I was going to have a continous plank from fore to aft.
This will give me the number of pieces needed. I would then add for waste. If I was going to plank end to end in one go, I would add 4 or 5 planks, in case of breakage.
If there were going to be butt joints, I would make sure that a plank could give me two pieces of different length that I could use. I would also add about 10-15% for wastage, from breakage or cuts that did not work out.
If I were going to curve the planking, I would allow 20% for wastage as I know I will break stuff.
I would expect to have stuff left over at the end, but this would go into my box of timber bits for future models. Much better than running out.