Boxed cake mixes are very forgiving-- packed full of emulsifiers and stabilizers-- they provide wiggle room in terms of over- or under- mixing and variances in oven temps; as long as you've got the correct wet to dry ingredients ratio. If not, here are a few ways to thicken your batter before it runs.
Cake batter should have a "dropping" consistency; this looks like the batter dripping smoothly and slowly from a spoon when tilted.
Cake Flour
One easy fix for adding too much wet ingredient is simply to balance it out by adding a comparable amount of dry ingredient. Shoot for a dry ingredient similar in texture to the boxed mix, like an equal combo of cake flour + confectioner's sugar. To do this combine 1/2 cup cake flour and 1/2 cup confectioner's sugar in a separate bowl and mix together thoroughly. Add the flour-sugar mixture to the cake batter, 1 tablespoon at a time, until the batter reaches a dropping consistency.
Instant Pudding
Cake mix ingredients are highly conditioned and ultra-processed-- that's why you don't need to sift them before mixing -- so is instant pudding. Thickening a runny batter with instant pudding has a couple of advantages: flavor and texture matching, plus no mixing, sifting or sweetening. Simply whisk in a matching flavor of instant pudding until the batter reaches a near-dropping consistency.
Tip: A little goes a long way. With instant pudding, you want to reach just shy of a full dropping consistency, as the batter will thicken more after it begins baking. Whisk in instant pudding 1/2 tablespoon at a time until a titled tablespoonful slides off after 1 to 1 1/2 seconds.
Eggs
Eggs thicken batter in two ways: (1) The protein-rich part of eggs, called the albumen, fortifies the cake's structure, giving it a firmer texture upon baking, and (2) the lecithin in the yolks emulsifies ingredients, creating stronger bonds between the fat, sugar, and flour. The fat in the yolks will also add moisture to the finished product. Beat 6 egg whites and 3 egg yolks into 1 batch of cake-mix batter.
Garnishes
Dredge garnishes in cornstarch. Cornstarch has twice the thickening ability of flour, and if the batter is slightly loose, 1/2 cup of nuts or dried fruit per batch dusted in cornstarch will do the trick. Add the dusted garnishes last; after the batter is completely mixed.