An ab initio molecular dynamics simulation based on combined quantum mechanical and molecular mechanical (QM/MM) potential was performed for Na+ in liquid ammonia. With this approach, many-body interactions that are very significant in determining the solvation number are modeled accurately by quantum mechanics. The method appears to resolve the discrepancy between many simulations that have predicted solvation numbers between 5 and 9. Although the solvation number of 5 obtained from our QM/MM simulation has not yet been confirmed by the experimental data, a derailed analysis of the simulation results together with a comparison of previous studies on known systems justifies this prediction.