How much sour cream should I add to a Duncan Hines cake mix?

https://www.cakecentral.com/recipe/7445/the-original-wasc-cake-recipe

The famous White Almond Sour Cream Cake WASC is a favorite in the Cake Central community. Here it is explained in detail how to make your own flavor variations and get the best results every time.

The original WASC, posted by MBalaska, is a tried and true recipe, beloved by the cake community of professionals and hobby bakers alike, was created by our own amazing kakeladi! 

well said @SandraSmiley !!

also when I need a quickie 2 dozen cupcakes and the stupid new smaller 15 oz.  boxes don't give me enough batter; I make a small extension~~~~~~ (a minor modification on Kakeladi's wonderful recipe !!)

use a box mix with all the  ingredients listed on box then ALSO  add:

1/4 cup all purpose flour

1/4 cup granulated sugar

1/4 cup sour cream

mix up  as normal with the box instructions, and this will give you 24 full sized cupcakes.

brilliance abounds 

That is a great idea, MBalaska!  I will remember that!  I do usually add the extra 1/2 cup per recipe of cake mix from a second box, just because I want the greater volume.

Yes, I used to add the extra mix also.  It's just easier and tastier to add the extra flour, sugar, and the delicious creamy sour cream.  You know how dreamy the addition of sour cream is, so it's a perfect fix.