There's probably about 2 board feet of wood in a LP junior, but that's not what you need to know.
The board feet you need to buy depends on the diminsions of the lumber and how it will have to be cut & glued to be a guitar. There can be a huge wastage factor, 2 or 3 times the final amount, so 4 or 5 board feet would be a reasonable guess.
A man hears what he wants to hear, and disreguards the rest. Paul Simon