someone posted yesterday about a company that did 1 Million TShirts for the Census Bureau in 25 days. DO the math on that! 16hr days, 25 days... that is 2500/hr
For 15 Million by Nov 30... June1 to Nov 30 is 183 days or so. At 7 days a week 16 hrs a day that is 5123 prints an hour. If you go 24/7 non stop with ZERO downtime (likely not possible) then it becomes a more managable 3415 prints an hour.
You will need 6-12 shops doing this job... or a guy with 20-25 automatics (the big ones)
May be better to print the image on the fabric roll to roll on high speed equipment... then cut & sew in overseas sweatshops. Even then you will need 200+ workers going 24x7 for 6 months to sew them if the can do 20pcs an hour.