This constant is the maximum number of vertex declarators for a mesh.
typedef enum { MAX_FVF_DECL_SIZE = MAXD3DDECLLENGTH + 1, } MAX_FVF_DECL_SIZE;
MAXD3DDECLLENGTH is defined as a maximum of 64 (see d3d9types.h). This does not include the "end" marker vertex element.
ID3DXBaseMesh, IDirect3DVertexDeclaration9::GetDeclaration, ID3DXSPMesh::GetDeclaration, D3DXDeclaratorFromFVF, ID3DXSkinInfo