Hypervalency of elements
First off, as suggested in the comment above, the contribution of 3d orbitals in hypervalency is probably minimal.

It's clear from sampling the inorganic literature that "beyond the octet," the size of the atom matters, as well as the shape of the ligands, size of the surrounding coordination sphere, etc. To some degree, it's a matter of fitting X atoms around the central atom.

If you have a 5th or 6th row element (2nd or 3rd row transition metal), lanthanide, actinide, etc., higher coordination is relatively common. For example, for lanthanide ions, 8-coordinate is the most common coordination number. Still, 8-coordinate iron complexes have been found, e.g. "Combined Mössbauer Spectral and Density Functional Study of an Eight-Coordinate Iron(II) Complex" Inorg. Chem., 2015, 54 (17), pp 8415–8422 or "Square-Antiprismatic Eight-Coordinate Complexes of Divalent First-Row Transition Metal Cations: A Density Functional Theory Exploration of the Electronic–Structural Landscape" Inorg. Chem., 2015, 54 (4), pp 1375–1383.

I'm not aware of a general overview of hypervalency and "what coordination numbers can you get", but there's a nice overview on modern descriptions using multi-center bonding.