Tetradecagon
Coxeter states that every parallel-sided 2m-gon can be divided into m(m-1)/2 rhombs. For the regular tetradecagon, m=7, and it can be divided into 21: 3 sets of 7 rhombs. This decomposition is based on a Petrie polygon projection of a 7-cube, with 21 of 672 faces.  The list A006245 defines the number of solutions as 24698, including up to 14-fold rotations and chiral forms in reflection.