For a better understanding of the answer given here, please go through the diagram in the attached file.
The diagram assumes that the base of the hexagonal pyramid is an exact fit (has same dimensions as the face of the hexagonal prism).
As can be seen from the diagram, the common vertices are A,B,C,D,E,F which are 6 in number.
The bottom vertices are G,H,I,J,K,L, which, again are 6 in number.
The Apex of the pyramid, P is one more vertex.
Thus, the total number of vertices in a Hexagonal pyramid is located on top of a hexagonal prism will be the sum of all these vertices and thus will be:
6+6+1=13