The question about how many medallion customers there are keeps coming up. I came across this line in a recent "Sky Priority" e-mail [1]:
"Sky Priority eligible customers fly 28 billion miles on Delta every year."
Since Gold/Plat/Diamond customers are eligible for "Priority", this can be used to guestimate the number:
Minimum miles required for Gold: 50,000: which would lead to 560,000 "Priority" customers. Given that a bunch of them are Platinum/Diamond, I would guess the real number is in the 400k range. At least an order of magnitude guess. Delta states that they serve 160 Million customers each year.
[1]
http://www.delta.com/traveling_check...rity/index.jsp