This is a space of vector valued Siegel modular forms with real representation $\mathrm{Det}^{ 18}\otimes\mathrm{Sym}^{ 20 }$. The following table gives the dimensions of various subspaces of $M_{18, 20}(\mathrm{Sp}(4, \mathbb{Z}))$.

Total39
Non cusp2
Cusp37