What are some of the worst rape crimes in world history during wartime?

That is indeed a terrible atrocity. It’s an estimated number by the way based on abortions, so the ones that were killed are not even represented.

Another source of war crimes like these was the Imperial Japanese Army during world War II.

Maybe you can’t call it worse, but it certainly lasted longer: all the way from 1932 until 1945.

Comfort women 

Comfort women were women and girls forced into sexual slavery by the Imperial Japanese Armed Forces in occupied countries and territories before and during World War II.[2][3][4][5] The term "comfort women" is a translation of the Japanese ianfu (慰安婦),[6] which literally means "comforting, consoling woman".[7] 

During World War II, Japanese troops forced hundreds of thousands of women from Australia, Burma, China, the Netherlands, the Philippines, Japan, Korea, Indonesia, East Timor, New Guinea and other countries into sexual enslavement for Japanese troops; however, the majority of the women were from Korea. [8] Many women died or died by suicide due to brutal mistreatment and sustained physical and emotional distress. After the war, Japan's acknowledgment of the comfort women's plight was minimal, lacking a full apology and appropriate restitution, which damaged Japan's reputation in Asia for decades. Only in the 1990s did the Japanese government begin to officially apologize and offer compensation.