So I am making this wedding cake for my sister in law:
in 13", 10", and 7" tiers. I found this guide for frosting amounts online:
AI think 25 cups would be about right since you are using buttercream for the filling too.
Just to double check - are you doing 4 layers of cake and 3 layers of filling per tier?
You could always make less and if you run out purchase additional ingredients to help with your budget needs.
I was going to bake up 2 pans of each size, cut those in half, and then have filling in between each one. So yes, I believe that would be 3 layers filling per tier. (had to pause and picture it for a second to figure that out! lol!)
AThat's a really usual cake....pretty but I think it would be so much prettier if it were all the rose swirls but each to its own....lol...it is still pretty just usual looking...