For some reason I want to say four, but not exactly sure. It seems like I've heard this somewhere at a cake class though.
LOL! thanks everyone...I think I'd have a huge mess if I put butter in my batter 
I'm still stressing on how to make a large cake and just going to do a test run....but I thought I'd ask first!
Not sure if this is what you're looking for or not. Thought I'd share it with you just in case. I got this off of another forum - these are NOT my own recipes, but I can't wait to try them. They were titled "DELICIOUS LARGE QUANTITY CAKE MIX"
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Butter Cake Recipe
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I used it for a stacked 14"square, 12"round, 8"square and 6"round wedding cake that a bride wanted. It held up beautifully. I frosted my cake with Earlene Moore's cream cheese icing and it turned out great.
This cake recipe is the maximum quantity for your 5 quart mixing bowl. It mixed up perfect in my Kitchen Aid Accolade. It yielded 15 1/2 cups cake batter.
2 Duncan Hines Butter Recipe cake mixes (I guess you could use any flavor)
2 cups All Purpose Flour
1 1/2 Cups Sugar
4 Sticks Butter (I used half butter, half margarine)
2 2/3 cups water
6 eggs
1 box French Vanilla Pudding Mix (instant)
2 tsp Baking Powder
Cream butter and sugar together. Add eggs, mix until blended. Add cake mix, pudding, flour and baking powder. Blend on low while drizzling in water in a thin stream. Mix until smooth (doesn't take long), and bake according to directions for your pan size. I had to share this because it turned our so good. I hope someone else enjoys!!
Earlene's cream cheese butter cream is a great tasting buttercream that crusts well. It stays soft underneath, and I like it because you don't have that heavy shortening taste.
This is her recipe:
Maximum recipe for a 5 Qt bowl
5 1/2 lbs. Powdered Sugar
3/4 Cup + 2 T Warm Water
1 T Butavan ( A thick butter, vanilla flavor, it's good, but when I don't have it, I use 1 1/2 tsp clear vanilla, and 1 1/2 tsp clear butter and omit 1 T of the water)
12 oz. warm cream cheese
1 1/4 lb. of Shortening
1 1/2 tsp. salt
She has mixing instructions on her site (earlenescakes.com), but I don't follow them. I just mix the shortening and cream cheese until smooth, and then dump in the powdered sugar and salt. I start my mixer on low and drizzle in water and flavor.
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Chocolate Recipe Version 1
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2 DH Dark Chocolate Fudge cake mixes
1 box chocolate fudge pudding
2 cups flour
6 eggs
2 tsp. baking powder
2 sticks butter
2 sticks margarine
2 cups sugar
1 cup oil
2 2/3 cups water
1/2 bag Hershey's special dark chocolate chips
I've done tiered and stacked cakes with this and it has been great. Also, if you put the two recipes together and swirl, you get a great marble cake.
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Carrot Cake
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OK...if anyone's adventurous out there, I have a new Carrot Cake version of this recipe. I'm doing another wedding cake for the daughter of a lady my husband works with and she wants carrot cake, so I did some experimenting and came up with this recipe. It turned out really well--the bride's mom told my husband that it was the best carrot cake she has ever had in her life. I'd never made a carrot cake before, so I was relieved to hear that!
Carrot Cake Ingredients:
2 Carrot Cake Mixes (Betty Crocker Super Moist)
2 cups flour
1 1/2 cups sugar
2 butter sticks
2 margarine sticks
2 cups water
6 eggs
1 box coconut cream instant pudding mix (small box by Jello)
2 tsp. Baking Powder
1 cup vegetable oil
1.5 tsp. Pure Orange Flavoring (Goodman's@Wal-mart)
1 pack of Gerber Tender Harvest Organic Carrots Baby Food (from 2-pack)
1/2 bag (14 oz. bag) Baker's Angel Flake Coconut Sweetened
3 tsp. Cinnamon
Cream butter and sugar together. Add eggs, and mix until blended. Add oil and mix. Add baby food, cinnamon, and coconut. Mix. Add cake mixes, pudding, flour, and baking powder to the bowl, but wait to mix. Put your water in a measuring cup and add the orange flavoring to the water. Stir the water and flavoring together. Then, turn the mixer on low and slowly drizzle the water/flavoring into the bowl. Mix until the ingredients look like they are combined nicely. I baked on 325 degrees and the cakes turned out great!
For icing, I used Earlene's cream cheese recipe and added 1 1/2 teaspoons of Pure Orange Flavoring (Goodman's) to it.
By the way, I have just under half a bottle of the orange flavoring left after making one batch of icing and batter, so I bet you can make two batches of batter and two batches of icing from one bottle of flavoring (1 fluid ounce).
I hope someone out there likes this recipe, too. Everyone at my house was fighting over the cake scraps because the flavor is so addicting...and we really aren't a carrot cake family...at least we weren't before this...
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Chocolate Cake - Version 2
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2 Duncan Hines Dark Chocolate Fudge Cake Mixes
1 Box Chocolate Fudge Pudding
2 Cups Flour
6 Eggs
2 tsp. Baking Powder
2 Sticks Butter or Marg.
1-1/2 Cups Sugar
1 Cup Oil
4 Cups Water
1/2 RECIPE
1 Duncan Hines Dark Chocolate Fudge Cake Mix
1/2 Box Chocolate Fudge Pudding
1 Cup Flour
3 Eggs
1 tsp. Baking Powder
1 Stick Butter or Marg.
3/4 Cup Sugar
1/2 Cup Oil
2 Cups Water
Cream Butter/Marg. oil and sugar together. Add eggs mix until blended. Add Cake mix, Pudding, flour and Baking Powder. Blend on low while drizzling in water in a thin stream. Mix until smooth (doesn't take long), and bake according to directions for your pan size.
Optional; fold in Hershey's Special Dark Chocolate Chips.
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