It's been revealed that they are making 140,000 units daily. If just one days worth is for Apple retail in the US it means about 130 iPhone Pluses will be available per store in EACH color, size, and carrier variety (assuming equal distribution in carrier, color, and capacity). Chances are big city stores get more and smaller city stores get slightly less.
If each unit produced for 3 days is designated for retail it would raise it to nearly 400 units of each type available. For all we know, stores are getting a weeks worth of production or more (as we don't know when production started but likely it was a month or more before when leaked parts started making their way online).
Given that people in line will want a variety of phones and carriers, I think you can be number 1,000 in line and still get one assuming you aren't picky as to what's left color or capacity wise.
Btw, precedent for this is the new iPad (3) when it came out. It was sold out online for launch delivery but anyone showing up to the store the day of even in the afternoon was able to get one (and usually the exact one they wanted).
Clearly they were saving units for retail to make sure people who physically spend time waiting in line are happier.