12NaBr is an ionic compound of Sodium (Na) and Bromine (Br), the question is asking how many total atoms of sodium there are. Because neither of the elements have subscripts, we know that the ratio for the compound is 1:1. This means that there is one sodium atom per NaBr compound. The coefficient, 12, tells us that there are 12 NaBr compounds. If there is one Na atom per compound and 12 compounds, then we know that there must be 12 atoms in total of Na.
