Thats derived from the number Congress worked out in 1866, Former SC judge sharkey gave to congress that 30% of all *****s were dead in Miss, in evidence when he and 5 other former USSC judges were sent south to find out if the south could feed irtself post war.
you can double check it from 1860 and 1870 cenusus data, start from 180k known fatalities, apply the 20% annual decade increase the ***** population increased by, compare to census data, and find that 3 millions of *****s are unacounted for, using not only lives lost but lives expected to be born and not,( nsame methodolgy as used to work our Shoah loss of life) Judge sharkey went to Miss who had a 23% increase average pre war, and a 0.1% increase in ***** population between 1860 and 1870. the highest loss of ***** lives, over 80% of the total, occur in the states of LA, Miss and Ga, which was where sherman and others of his kind waged war on civilians and the land itself.
You can read about in such books as A Constitional history of secesion, and war crimes against Southern civilans.