In the chemical equation Cu2+ (aq) + 4 NH3(aq) → Cu(NH3)42+ (aq), there are four NH3 molecules involved because copper(II) ion (Cu2+) has a coordination number of four. This means that it can form four coordinate bonds with other ligands, in this case, NH3 molecules. The four NH3 molecules donate their lone pairs of electrons to the copper(II) ion, forming a complex ion called copper(II) tetraammine.