Just wanted to note that Jeanne’s batch size is from the wedding cake section, p. 514, it uses 2.5 pounds of butter to make 11 cups of frosting.
I’ll add something on the smaller batch size (one pound of butter or 4.5 cups of mousseline), because my 6QT KA mixer won’t handle the 11 cup batch, it can only handle about 9 cups of mousseline at a time.
For Rose’s mousseline, I have in my notes that one cup frosts 8 cupcakes, so 36 would need about one batch (4.5 cups of mousseline, using one pound of butter).
For the half sheet cake, Rose’s chart on p. 513 of the Cake Bible suggests that you would need 6 cups of frosting for the top and sides of a single 12x18x1.75 layer. If you’re going to make two of those and fill between the layers, you would need double that amount, or 12 cups. That chart does not include decorating, so if you’re going to pipe edges, etc., you might want to include another 3 cups. In all, cake, filling and decorating, it might take 15 cups of frosting, or about 3 1/3 batches.
So for both projects, 19.5 cups of mousseline or about 4 1/3 batches. This is very close to what Jeanne is saying, 22 cups.
Hope you have a humongous mixer