12.0g x 1 mol / 63.546g = 0.188839581mol
So, for every 1 mole, we have 6.022 x 10^23 of whatever we're measuring. This gives us a conversion factor of (1 mole / 6.022 x 10^23 atoms) or (6.022 x 10^23 atoms / 1 mole).
0.188839581 mol x (6.022 x 10^23 atoms) / 1 mol = 1.137191955 x 10^23
Remember from before that we are limited to 3 significant figures. Since our calculations are complete, we can now round down to: 1.14 x 10^23
That should be your answer!
Hope it helps!
xo