We have investigated the structural properties of Na+ in 18.45% aqueous ammonia solution by performing a combined ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulation. The first solvation shell of Na+, treated by Born-Oppenheimer ab initio quantum mechanics using the LANL2DZ basis set, contains 3.7 water and 1.8 ammonia molecules, in contrast to the corresponding values of 4.9 and 2.2 obtained by classical pair potential simulation. The results show the effects of many-body interactions on the geometrical arrangement as well as on the coordination number of the solvated Na+. The relative preference for ammonia molecules is discussed in terms of the chemical concept of "hard" and "soft" ions and ligands.