https://apholt.com/2019/01/30/death-estimates-for-the-crusad...
John Shertzer Hittell– Estimates 1,000,000 total dead for the crusades to the East covering the period from 1095 to 1291.
“In the two centuries of this warfare one million persons had been slain, but it had not been without some compensations.” John Shertzer Hittell, A Brief History of Culture (1874), p.137.
[...]
John M. Robertson– Estimates 9,000,000 total dead for the crusades to the East covering the period from 1095 to 1291.
“It is a reasonable calculation that in the two centuries from the first crusade to the fall of Acre (1291) there had perished, in the attempts to recover and hold the Holy Land, nine millions of human beings, at least half of them Christians. Misery and chronic pestilence had slain most; but the mere carnage had been stupendous.” John M. Robertson, A Short History of Christianity (1902), p. 278.