It is easy to account for ion formation for the alkali metals. These are supposed to have only the 1 valence electron, which is lost after it oxidizes a non-metal: cf NaCl,KCl,CsI etc. These metals, upon oxidation, have lost all their valence electrons (the which means the outermost electrons). Transition metals have valence electrons amongst the s shell and the d shell. Electrons from the d shell are less penetrating (i.e. and as a consequence less attracted by the nuclear charge), and multiple oxidation states are possible: cf. Fe(I), Fe(II), Fe(III), and even further oxidation states. We remember that oxidation numbers are a formalism, but they are still useful to track metal reactivity.
Most transition metals, Fe, Cr, Ni, for example, have multiple states (including, curiously, negative ones and ones with zero oxidation states). Oxidation states are our attempts to categorize and classify this chemistry.
