A BUNCH! Sorry Ray, I could not help myself. I think for many situations the bigger issue would be the heads ability to flow air. The next issue would be the stack length as longer stacks struggle on the top end due to the length of the column of air that has to start and stop moving all the time. But then again, I am just babbling about stuff that you already know so I will shut up now. (If I had any useful info I would share it, but alas I don't).
Paul