Scott Bourne (Apple Phone Show, MBW) is stating 20 per AT&T store, and he is pretty reputable.
As much hype as their is surrounding this product, I don't know if the camp-out lines are going to happen. I can see it both ways though.
Unlike the PS3/Wii, the iPhone is not an evolution product (unless it is evolving from the iPod), and it is $500 + Contract + Data Fees at the MINIMUM. This is not like when console owners went to go "upgrade", and were able to plop down their cash and that was it.
I'm nervous either way, for starters - I'm wondering how reliable these AT&T stores are going to be. Will I really be able to walk in and buy the day of? No shipping errors at the store or late deliveries?
Aside from that, since the AT&T store is only about 3 miles away from me, I'll drive by around 9pm, 12pm, 2am - until I see if a line is even starting. I'd feel a lot more secure in the fact of taking one home same day if I were at an Apple Store, but I'm almost positive their will be a ridiculous line.
And if Apple has any sense, they are going to under-ship these units. I want all of us Mac lovers to have one - but it is going to lose its cache if someone can walk in the following Monday morning and buy one of the twenty they have on the shelf. As great as this product is, Apple needs to keep the level of "wanting it" up for a few months because of its steep price point.