I'm assuming your stock is long enough to get full length strips from and that you're going to use 1/4" x 3/4" cove and bead strips. If your saw cuts an 1/8" kerf about a third of your wood will wind up as sawdust, which gives you about 20 strips per 1x8. With a cove and bead each strip covers 5/8" of the girth of the boat. A conservative girth is about 60". Divided 60 by 5/8 and you wind up needing 96 strips. Five boards will let you pick and choose to match color and grain.

As far as getting longleaf pine, I'd plan on going to either a custom sawmill or a specialty wood dealer, unless you are confident that you can identify the wood when you see it. I also seem to recall that sometimes longleaf pine is available as salvage wood from old buildings.