This is a space of vector valued Siegel modular forms with real representation $\mathrm{Det}^{ 12}\otimes\mathrm{Sym}^{ 22 }$. The following table gives the dimensions of various subspaces of $M_{12, 22}(\mathrm{Sp}(4, \mathbb{Z}))$.

Total18
Non cusp2
Cusp16