1- 15 000 000 its for volume 1-18 + novel + spin-off 4-koma + spin-off vigilent.
2- As of December 31, 2017, the total circulation of My Hero Academia (excluding spin-offs and novels) was 11,843,000 copies.
3-
December 7, 2015 to November 28, 2016 : 5,114,916 (sales) for My Hero Academia (excluding spin-offs and novels)
December 5, 2016 to November 27, 2017 : 5,852,310 (sales) for My Hero Academia (excluding spin-offs and novels)
December 4, 2017 to May 28, 2018 : 2,516,768 (sales) for My Hero Academia (excluding spin-offs and novels)
So :
In novembre 27 (2017) : 5,114,916 + 5,852,310 = 10,967,226 (sales) for My Hero Academia (excluding spin-offs and novels)
In may 28 (2018) : 10,967,226 + 2,516,768 = 13,483,994 (sales) for My Hero Academia (excluding spin-offs and novels)
So no, the figure of 15 million isn't weird.