White cake mix 5lbs bag making only one cake

Last thoughts, most boxed cake mixes are just over a lb in weight...so maybe divide your bulk 5 lb package into 5 containers, and use them one by one as you need to bake.

Another way to go - if this is a branded product, look for a recipe or directions on how to use at the manufacturer's website.

Have a look at this recipe (or others) for homemade white cake and adjust your use accordingly. Here, about 4 and 2/3 cup dries (flour, sugar, baking powder) mix with fat, milk, eggs and vanilla for a two layer cake - often the planned size of a cake from one box of mix.
And/or look at some other ideas of what to do with cake mix, since you seem to have received a few months' supply.
https://sugarspunrun.com/the-best-white-cake-recipe/#wprm-recipe-container-13187
https://spoonuniversity.com/recipe/things-to-make-with-cake-mix-that-aren-t-cake