ANSWER: You are correct that the Uniform Bylaws provides that “[a] majority of those currently serving on the board shall constitute a quorum.” Article 5, Section 9(c). As defined in the glossary at the back of the Bylaws, “majority” means more than half. In your local unit’s current situation, with twelve actual people serving on the board, a quorum would be more than half of twelve, or seven. If/when all positions are filled by different individuals, a quorum would be more than half of fifteen, or eight.
Finally, when one or more positions are shared, such as in the example you give, each of those individuals are “serving on the board” and would be counted in determining a quorum.