# How many atoms in TWO formula units of MgCl_2?

I think you mean $\text{how many atoms.......}$
I think you mean $\text{how many atoms are present in 2 molecules,}$ $\text{or better 2 formula units of}$ MgCl_2?
And clearly there are 6 atoms present , i.e. $4 \times C l + 2 \times M g = \text{6 atoms in total}$.