This is a space of vector valued Siegel modular forms with real representation $\mathrm{Det}^{ 12}\otimes\mathrm{Sym}^{ 28 }$. The following table gives the dimensions of various subspaces of $M_{12, 28}(\mathrm{Sp}(4, \mathbb{Z}))$.

Total32
Non cusp3
Cusp29