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).

Do you know him?

We've met a few times, but I learned this when I was escorting him around the Bay Area for a day during his book tour.

Yet he has one finger on the pulse of the internet-nation, as evidenced by the previous comic: http://xkcd.com/1492/