Make AlMgSi-system LAMMPS file using ASE
A bit Math first To get $y$-at%Mg2Si for the 1.2wt%Mg2Si system, calculate the following simultaneous equations,
$$\begin{aligned} \frac{(\text{Mg$_2$Si})_y}{\text{Al}_x +(\text{Mg$_2$Si})_y} &=& 0.012, \ x + y &=& 1, \end{aligned}$$
using masses, Al=26,981, Mg=24.305, and Si=28.085. We get $y\simeq 0.00425$.
In case of 32000-atom system If we put (20,20,20) cubic FCC cells including 4 atoms in the simulation box, the box contains 32,000 atoms. In the 32000-atom system,
$$32000*y \simeq 32000 *0.00425 \sim 136,$$