Okay, so I'm renting a shared commercial kitchen and when I'm doing wedding cakes I find myself still only doubling my cake recipe because I've heard you can't triple or quadriple it.

How do you pro's make large quantities of cake from a scratch recipe?

I do... I go up to 5x with most of my commonly used recipes. Only a few of my more finicky cake recipes need adjustments when they get up to 3 and 4 times.

I sure do also. I would die if I had to make one batch at a time. Like PinkZiab said, only some would be a pain after 3x or 4x, like (for me) chiffon or sponge cakes, and one of my white cakes just hates me and falls if I more than double it, otherwise I do as many as I can in my big mixer.