Coordination unsaturation is effectively the maximum coordination number that a metal ion can adopt.
Coordinative unsaturation is based on ionic radii (ultimately atomic radii from 0.1), where the atomic size increases from right to left (lower nuclear charge) and top to bottom (increasing n level being filled). For any metal, ionization readily occurs to generate a cation, which will yield ionic radii smaller than atomic radii. In this case, the metal cation is considered a Lewis acid with Lewis basic ligands surrounding it through coordinate bonds. The number of Lewis bases surrounding the Lewis acidic metal cation is the coordination number.
The metal cation charge does complicate comparison of say K+ to Ca2+, where Ca2+ is smaller despite being to the right of K+. In this comparison, the coordinative unsaturation is roughly equivalent between K+ and Ca2+ because the additional charge on Ca2+ compensates for the decreased radius to yield both ions tending up to a coordination number of 8.
So the coordinative unsaturation increases as you go down a group: Be2+, 4; Mg2+, 6; Ca2+, 8; Sr2+, 8, Ba2+, 8.