Actually. he rarely has comics in reserve. He has a notebook of ideas and sketches, and each day a few hours before a comic is set to go up, he picks one and draws and inks it. I've watched the process firsthand, and I was surprised as to how little lead time there is.
When he knows he'll be busy he gets a few in the hopper, but it isn't his standard SOP (at least it didn't used to be).