What Anne says: You would want to have enough pans to bake that many at one time, and enough oven space as well. What size mixer do you have? How much oven space do you have?
Having said that, our biggest single cupcake order this holiday season was for 2,500 mini cupcakes. When I moved into my new space last fall, I bought a new double stacked Blodgett convection oven. Even with five racks in each oven, and 10 pans of 24 each, the most I could bake at a single time was 240 cupcakes. And then I had to consider that while the cupcakes were cooling, I couldn’t just put another 10 pans in the oven because I only had 8 pans of 24 to begin with. So I bought 12 more pans (and they weren’t cheap. I fussed about it for days - $684 on mini cupcake pans! seriously? I could have spent that easily on so many other things I want for the bakery!). It took 3 solid days of baking to get that order done. In addition to all the other stuff we had going on that week.
So you want to consider that although you might get an order or have multiple orders which call for 400 cupcakes (and during wedding season, it’s not unusual for that to happen on a weekend. Two weddings of 200 people and you’re there.) you are going to make batches of batter and bake off as many as you have pans for, so you might only need to scale it up by a factor of 4 to make 100 at a time.
For a 20 quart, the biggest batch size for an Italian Meringue buttercream uses 6 pounds of butter. In school, there was an 80 quart mixer that could handle something like 25 pounds of butter, I think. It was enough for 8 three tier weddings cakes to be filled and finished - that’s all I can remember!