i would do what's easiest for you! i think a chafing dish or something to keep it warm is a good idea, unless everybody is going to sit down and eat and finish all at the same time... all i know is hot pasta is by far the best kind of pasta!

i don't know if you should judge amount by # of serving dishes... how much pasta/mac and cheese fits in one serving dish?? one pound? a pound and a half?

i would probably want to cook about 4 lbs of pasta for 25 people... knowing me, maybe more just to be sure!

I would bake three individual pans and use them one at a time in your copper holder. That way, when one starts to look a little sparse, you can quickly switch out to a new pan. I can't help you with baking instructions, but once the casseroles are cooked, you can cover them with foil and keep them in the already warm oven that has been turned off, but that still retains some heat. Lightly spray or oil the foil so that it doesn't stick to your nicely browned casserole.

I often bake two 9x13 casseroles (ziti, mac and cheese, scalloped pots etc) at the same time. I rotate the pans and switch racks halfway through.

I like the idea of individual pans, because you can swap them out, as Fun2B suggested. I also love the crispy edges of Mac and Cheese, so definitely serve from the baking dish.