A stitch and glue boat will typically use three or four sheets of 4mm Occume marine plywood. With careful cutting you may be able to be more efficient.
For a strip-built kayak a good rule of thumb is 2 board feet of western red cedar or other softwood for each foot of boat length. A board foot is a volume of wood 12" long x 12" wide by a nominal 1" thickness. Most wood you will buy is planed down to 3/4" thick or even 11/16" for some cedar and redwood.
To get a little bit more specific it takes just under 18 strips to cover a 12 inch wide area. In other words to completely cover one square foot of surface area on a boat you will need 18 linear feet of strips. If you are looking at a design with a surface area of 50 square feet you will need approximately 900 linear feet of strips. This does not leave much room for errors so you will want to include some extra.
You can typically get three 1/4" thick strips out of each inch of board width for a yield of 36 linear feet of strips per board foot. This works out to 1/2 a board foot of lumber per square foot of boat surface area. So for 50 square feet of surface area you would need a minimum of 25 board feet of lumber.
Notice that the surface area of a boat varies widely for boats of the same length; the Petrel has a surface area of 46 sq ft and the Great Auk is 59 sq ft. The rule of thumb would suggest 34 board feet where the Petrel only needs 23, but the Great Auk needs almost 30, so the rule generally over estimates, but you will probably happier to have too much wood instead of too little. Don't try to get exactly the right amount forcing you to use tiny scraps to finish covering the boat.
You will also want a small amount of hardwood to finish up the boat. The coaming lip can be made by cutting up scrap strips into thin laminations, but you may want some nice hardwood such as ash, cherry or mahogany. Outer stems can also be constructed of hardwood. One board at least 36" long x 6" wide should provide you enough for most purposes.