They have no way of knowing that number. They only way they could is if they don't sell in stores at all but rather make you order online and then they build it and ship it to you.
Now you will say they should have some idea and just triple that to be safe. Trouble there is that they can't trust folks not to leak parts. You really think they believe no one would get greedy about selling the location of the warehouses where the stockpile goes. They can't take that risk. And they don't have stores big enough to store them there (plus the same security issues).
In the end, having to perhaps wait a little longer for your iPad cause of high demand is a first world problem. Whether Apple doesn't put in a fancier screen because the suppliers can fill he orders etc, the wait will still happen. Prepare yourself and when you are all depressed cause it's been a month and you are still stuck with your iPad 2, go volunteer at a Battered Women's Shelter. Seeing how they are scarred for life etc will give you a sense of perspective