This work deals with a 2D numerical study about the effect the number of inlets of a micromixer on the mixing efficiency when two fluids must be mixed at low Reynolds and high Schmidt numbers. In this study, 6 different inlet configurations (with 2, 3, 4, 5, 6 and 7 inlets) for 2 Reynolds (0.1 and 0.29) and 3 Schmidt numbers (10 x 10(3), 4 x 10(3) and 2 x 10(3)) have been considered. Just as reference micromixer and for comparative purposes, the geometry studied in Ortega-Casanova (2017), with just 2 inlets, has been now redesigned with more inlets incorporated. Originally, the micromixer with 2 inlets, Re = 0.29 and Sc = 10 x 10(3) gave a mixing efficiency of similar to 14%, while we show in this work that with 7 inlets and the same Reynolds number, the efficiency can be increased up to similar to 80%, when Sc = 10 x 10(3), and up to similar to 95%, when Sc = 2 x 10(3). Later, we show that the mixing cost of the optimal inlet configuration is one of the lowest in comparison with previous works. Additionally, when the optimal configuration is used as microreactor, the reacting efficiency also increases in comparison with the 2-inlet configuration. We also estimate the number of inlets for full and complete mixing.