To explore the coordination number ( around the cation) dependence of the nonlinear optical (NLO) properties in alkalides, this paper studies the structures and large NLO responses of model alkalides, Li(NH3)(n)Na (n = 1-4). At the MP2/aug-cc- pVDZ level, the structural characteristic is determined to be that the Li-Na distance increases ( from 3.030 to 4.646 angstrom) with the increasing of the number of NH3 (n from 1 to 4). Results show that Li(NH3)(n)Na ( n = 1- 4) have considerably large first hyperpolarizabilities (beta(0)). Especially, a prominent coordination number dependence of the beta(0) value is found as follows: beta(0) = 13 669 ( n = 1) < 26 840 ( n = 2) < 39 764 ( n = 3) < 77 921 au ( n = 4) at the MP2 level. With the same coordination number ( four N atoms) of Li+ cations, the beta(0) value (77 921 au) of this "small" inorganic molecule Li(NH3)(n)Na is over five times larger than that of the "big" organic molecule Li@ Calix[4] pyrrole-Na (14 772 au). This indicates that the beta(0) value is strongly related to the flexibility of the complexant. Obviously, the flexibility of ( NH3) 4 is much greater than that of the cuplike shaped Calix[ 4] pyrrole. This work suggests that two important factors should be taken into account to enhance the first hyperpolarizability of alkalide, i.e., the coordination number around the cation and the flexibility of the complexant.