To find the sum of 18 and 12, we calculate:
\[ 18 + 12 = 30 \]
In the context of the problem, this sum represents the total amount of the cinnamon and sugar mixture that Micah is making. Specifically, the recipe uses 18 cups of cinnamon and 12 cups of sugar, resulting in a total of 30 cups of the cinnamon-sugar mixture that can be sprinkled on toast.