The Taiping Rebellion was a major millenarian religious movement in 19th century China that seized control of a fair part of Southern China and fought a war with the Qing Dynasty. It occasionally comes up on Reddit as an example of an extremely bloody conflict the importance of which is underestimated. The figure for the death toll which is often cited here is 20 million people total, which seems to come from this wikipedia article.
This strikes me as an extraordinary claim, for a couple reasons. First, 20 million people is a lot of people: the upper bound for all deaths during WWII including deaths from disease and famine, and the Holocaust, is about 80 million.
Second, there's the question of who was doing the statistics and demography. The Qing dynasty was not politically stable, obviously, and had reasons to exaggerate the death tolls.
Third, there are reasons to continue to do so. A 19th century Christian religious movement in China is not going to have many defenders, least of all on Reddit. Conversely, the CCP today has reasons to emphasize the destructiveness of the conflict in order to underscore the importance of controls on religious freedom, much of which are in fact directed at designated 'cults' like Falun Gong. There's also a tendency in official Chinese historiography to cite the extreme upper bound of death toll estimates as settled facts, sch as with the Nanjing Massacre.
As against this, it's not totally clear who compiled this statistic. According the wikipedia, Mao also praised the Taiping Rebeliion. The wikipedia article where the number comes from simply cites the Encyclopedia Brittanica, which is difficult to evaluate as a source.
Third, there are reasons to continue to do so. A 19th century Christian religious movement in China is not going to have many defenders [...]
You'd think so, but it does, or did at least. I recently got my hands on a little, English language, book on the rebellion, published in mainland China in 1976. Its view of the rebels is quite favourable. It downplays the religious aspect of the rebellion (their version of Christianity was extremely heretical at best anyway) and presents it as a revolutionary peasant movement, fighting against feudal oppression and foreign imperialists; something the Communist Party was quite in favour of. To give you an idea, the title of the last chapter is "The Heroes of the Taiping Heavenly Kingdom Will Never Be Forgotten" (yes almost every word starts with upper-case). One of the final pages has this paragraph
[...] Although the Taiping Revolution failed, its marvellous contribution to the forward march of history will last forever.
EDIT: The book is called The Taiping Revolution, published by the Foreign language press, no author is named.
So this is according to Chinese google. It did turn up one or two formal studies but that's it.
I'm going to summarize one of them.
20 million is the estimate from the American consulate(?) at the time. An American missionary working from the front actually give a far larger number of 60 to 80 million.
We do have some census figures. Qing dynasty took household figures and we can estimate from that. At the time of the rebellion in 1851, the population covering the five provinces of Hubei, Anhui, Jiangxi, Jiangsu, Zhejiang, number about 170 million, roughly 40% of the population of China at the time. These five provinces are the main battlefields, and also amongst the most heavily populated provinces in China (the mid and lower Yangtze). We do have census figures every few years that show drastic changes. The 1911 census list only 105 million. Taking into account the population growth after the rebellion would actually bring us to to a death toll of 70 to 90 million. So perhaps the Jesuit wasn't that far off.
So a lot of Chinese scholars give the number at 60~90 million.
The article does point out a lot of problems.
First, the rebellion itself completely destroyed Qing's ability to take census figures in those provinces, so they should be taken with a pinch of salt. There's quite some drastic changes in the census figures in those years that could be taken as government unable to take figures during war accurately, and after the war population settling back down and moving back in.
Second, besides the Taiping, three major things happened between 1851 and 1911 that effected the population. 1) The Second Opium War. 2) The flooding of the Yellow River in 1855, silting up of the Grand Canal, and beginning of transporting by sea and rail led to a lot of economic hardships in the area and of course death. 3) The Boxer Rebellion.
Third, if population growth after the rebellion remained the same, that must mean population density did not fall below certain threshold (though immigration from other provinces after the rebellion helped).
Fourth, the Qing army totaled 800 000. Even after considering Taiping numbers (which was smaller) and local militias, each soldier would still need to kill dozens of men to make up 60 to 80 million.
The article concludes with the age old "more research is necessary".
I should note here that the author's criticism is directed towards 60~80 million, not 20 million.
Now looking at that number, here are my personal opinions.
Like the Mongol Conquests and Thirty Years War, most of the depopulation is more likely due to disease, starvation, and exposure as people flee, farms and food stores are destroyed, and the government having no ability to help.
Combining a civil war, total war, and holy war, the Taiping Rebellion is absolutely devastating and massacres were often.
60 million would amount to over 35% of the local population. By contrast 20 million would be under 12%. Considering the estimated death toll of the Thirty Years War, 20 million is not extreme at all, and 60 million is not impossible.