While the bulk of my knowledge lies in Chinese literature rather than history, I can provide some insight (although not the concrete answer you're looking for).

Lu Xun's Diary of a Madman is one of the most influential texts in communist China after the little red book. The premise is that a man begins to see the words "eat people" between the lines of Confucian texts. It was one of the main works criticizing the old culture, fueling its fall and eventually the Communist revolution. Revolutionaries of all stripes were profoundly influenced by Lu Xun, and it was essentially required reading in the early days of Communist China.

Many in Mao's China saw the parallels, and Lu Xun was used as a disguised critique of the CCP to the point that this foundational work was eventually banned.

The point is, that while cannibalism may have occurred, it is likely overblown due to the symbolic significance of that particular act in revolutionary China, as any critic could use it as a semi-coded shorthand for a degenerate society.

Short answer: Somewhere between 20 and 43 million people died in China during the Great Leap Forward and Cultural Revolution. Did, at some point, people cannibalize? Absolutely yes. It is well documented.

Two sources on cannibalism in Modern China are the classic, but potentially overdramatized Scarlet Memorial: Tales of Cannibalism in Modern China by Yi Zheng, and Donald S. Sutton's academic paper " Consuming Counterrevolution: The Ritual and Culture of Cannibalism in Wuxuan, Guangxi, China, May to July 1968", published in 1995. Note that Sutton uses Zheng's accounts as a prominent source in his paper as well.

The Statistics

In his book, Zheng (who, in 1968, is observing the Guangxi province as a Red Guard stationed in Wuxuan County) lists that in Wuxuan County itself, "one hundred and several tens" were victims to cannibalism. The official government number is 64.

In Guangxi Province as a whole, Zheng estimates that 100,000 people ate human flesh in Guangxi in the early summer of 1968 with a total victim count of 1,200; however, Sutton contests this figure as arbitrarily high. Zheng goes into gruesome detail (as qtd. in Sutton 1995 from Zheng 1993: 96):

Fifty-six had their heart and liver cut out; 18 were completely consumed (down to the soles of their feet), 13 had their genitals eaten, one was decapitated after being eaten, and 7 were actually cut up while they were still alive.

Zheng includes a list detailing the precise locations in Wuxuan County of the location of the attacks, which Sutton includes in his paper as a map (I can PM anyone the PDF if interested).

For example (copying from Sutton's table of victims):

Date	Place	Victims	Type	Methods	Parts Eaten	How Disposed
May 4	Tongwan	2 Tans	Struggled	Shotgun	All flesh	Distributed
June 18	Wuxuan	Wu	Struggled	Beaten	Heart, liver, thigh	School Banquets
July 10	Mashan	Diao	Fugitive	Shot	Heart, liver	Hot pot by Militia
July 17	Sanli	2 Liaos, 2 Zhongs	Struggled	Clubbed	All flesh	Eaten by 20-30 at Brigade HQ

Pretty gruesome, eh?

The Cannibals

It is interesting to note that both Zheng and Sutton describe the Wuxuan cannibalism as not an act of desperation due to starvation or famine; in fact, essentially all of the cases were perpetrated as acts of political vengeance.

As Guangxi province and Wuxuan especially objectively one of the most brutal sites of Cultural Revolution infighting, the rival Party leaders encouraged their supporters and local intelligentsia (everyone from fiery Red Guard youth to old women to schololteachers), over a six week period, to attack political enemies and consume them.

The killing of political rivals through cannibalism was seen as the ultimate punishment: it combined all aspects of archaic Chinese punishment (which were brought back in full during the Cultural Revolution): the victims were denied the filial obligation to ancestors to keep the physical body in one piece; the victims were paraded about like the comical satire of bad behavior; the victims, guilty of high treason, were ridiculed and eventually reduced to simple pieces of flesh and meat.

A particularly succinct example (taken from Sutton):

A variant was the parade of body parts. Thus, after the military defeat of the Small Faction, Zhou Weian, its captured leader, was executed and his head and legs taken first to Luxin village as a sacrificial offering at the memorial meeting for two of the Big Faction members and then to the county seat for theatrical use in a cruel catechism with his pregnant widow (Are these your husband's head and legs? Was he a bad person? Is this your husband's thigh bone?)

Perhaps Sutton puts it best: "The popular sense of justice required that punishment fit the crime and that no punishment was severe enough for an old feud...Cannibalism was an extension of the same idea. To chop up, cook, and masticate was still more complete way of offending bodily integrity, depriving the enemy of humanity by reducing him to the status of a comestible."

Oh, by the way, eventually the Cultural Revolution ended and the new reformist government took power after the ousting of the Gang of Four. The cannibals were punished by anything from fourteen years in prison to expulsion from the Communist Party.