We don’t put a limit on it. If we’re at a store, we abide by their policies, so check with the managers. If we’re at a con, there’s usually not a limit—check with the show.
If it’s up to us? No limit. Basically, how many do you feel like carrying around with you all day? If you bought it and brought it, it seems the least we can do to sign it. Couple of things you can do to make our lives easier, though: 1) take your books out of the plastic bags BEFORE you get to the front of the line pleeeeeease; and 2) if you have more than, say, 30 books or so, let us sign those 30, then go to the back of the line. Feel free to do that as many times as you like.