Alea iacta est! (The die is cast!)

There are five faces that are horizontal and hidden: the bottom face of the top die, and the top and bottom faces of the other dice. Since on normal dice the number of dots on two opposite faces adds up to seven, there are two times seven, or fourteen dots hidden on the horizontal faces of the two bottom dice. The top die has seven dots in total on its two horizontal faces as well, minus the one dot that is visible, equals six. So in total there are fourteen plus six, or twenty dots hidden on horizontal faces.