A clock face is a sort of model regular convex dodecagon. The vertices are the clock numbers 1-12. A diagonal is formed by joining opposite vertices (12-6, 1-7, 2-8, 3-9, 4-10, 5-11) making 6 diagonals. This only applies to a regular or symmetrical dodecagon, otherwise opposite vertices may not be aligned. So the maximum number of diagonals is 6.