Typically, there are no extra consoles on launch day. There are almost never any available unless you've pre-ordered.
But Sony has said in the past that they've been "holding back" some PlayStation 4 units; in other words, they can't be pre-ordered and they'd be available for regular consumers on day one. In checking with retailers like GameStop, that does appear to be true.
Best Buy has stated that they prefer to see gamers who didn't pre-order at their midnight launch events, which obviously means they've got extras. And after talking to a few local GameStops, it seems they'll have some extra units, too. I couldn't get an exact number from any store (it was implied that they didn't know the exact extra count), but it seems that at least "some" PS4s will be available to those who didn't pre-order.
Of course, those will go on a first-come, first-served basis, so you had best get there pretty early. GameStop midnight launch events will begin at around 6 p.m., so maybe you should get there around then and grab your spot in line. The fact that there are any available at all is quite remarkable, I say. Historically, if you didn't pre-order when you had the chance, you could be out of luck for months .