According to Wilton, you need 7 1/2 cups of batter for each 12" round 2" deep tin
I find that you have to know how each cake recipe bakes. Certain cake recipes bake higher than others so sometimes it's trial and error. Frankly I don't think 7 1/2 cups is enough for a 12" pan for most recipes. Your cake may be a little flat. Usually filling pan 2/3 full is a pretty good bet.
Also you'll want to use baking strips or a flower nail with that size pan.
When I did a cake a few weeks ago involving the 12" round tin, I found using about 8-1/2 cups of batter generated the right height I needed for the cake. I know Wilton says 7-1/2, but when I was baking, I realized 7-1/2 cups didn't fill the tin as much as the other size tins I was using. If I only used 7-1/2 cups, my largest (bottom) layer would have been shorter than my other layers. I think you have to play with it, depending on the recipe.
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