Tbish, I’m not sure if you are talking thickness or width of the stock blank, but I think you mean width. The required width of a plank to be cut into a stock blank all depends on your stock pattern or blueprint and the orientation of the grain. Must start out with a blueprint or plan.
Some late English trade guns were made with so little drop, probably to save on wood, that it would be possible to fit the pattern in a lumber yard 2x8. Guns with deep buttplates and a lot of drop need 10” of plank width and that’s without trying to align the grain in the wrist.