Lets do a simple math with what we know.
Foxconn daily production of iPhone 6+ : 140,000
25 days of production for launch day
140,000 x 25 = 3,500,000 total production x 0.96 for DOA = 3,360,000
3,360,000 split between 18 countries.
For the hopeful, say apple holds back 70% stock for the US market = 2,352,000 million
2,352,000 / 4 major carriers = 588,000 per carrier
588,000 x 0.5 half preordered = 294,000 total for first day sales
294,000 / 257 total apple stores = 1,144 total phones at an apple store available
1,144 x 70% held for retail stores = 343 per apple store / 4 carriers = 86 phones per carrier
Now if apple only saved 50% for the US market, number shrinks.... Make your choice when thinking this is like the iPhone 5s release in availability.
Interesting math, but a lot of assumptions we just don't know. For example, the amount allocated for preorders might have been less than half. Also we have no idea how many days it has been in production. Part leaks have been occurring for months, so for all we know they started a long time ago.