I have alway understood that it was fully voluntary and "at own expense" as a board member, and I trust that the previous posts are correct.
Further, I understand the large size of the board is to give a broad spectrum of members. NRA is so large and into so many things that it takes a lot of people to touch all areas and all members. Another benefit of size, even if it wasn't intended, is that if a group tries to infiltrate it will be tough to get a foot hold or majority.