So according to Wikipedia, only 12% of China's land is arable, which has led to many famines throughout China's history. However, ancient China was also very wealthy and populated compared to other regions of the world. How did China get its wealth and large population if it had so little arable land?
Short answer:
Don't confuse "percentage of" with "amount of". China could (and can) produce huge amounts of food and was doing so in prehistory.
Discussion:
China has a tremendous amount of arable land, and China begins farming it very intensively -- wet field agriculture typically produced more calories per acre than early dry field techniques-- in the Neolithic. The Wikipedia factoid you're citing is deceptive-- China _today_ includes a vast amount of thinly populated mountainous and desert land to the West, regions that weren't part of China in ancient times. If there are a million hectares of desert and a hundred thousand of high quality arable land-- all you need know about is what was happening on the _arable_ land. That there was a lot of mostly empty desert is irrelevant, then and now. Similarly, to understand US agricultural productivity, you need not concern yourself with Alaska's mountains and frozen tundra -- you look at California's Central Valley, Iowa, etc.
So what you want to know is not "what percentage of the greatly expanded China today is arable?" but rather "how much arable land was there, in absolute terms, and how much were they able to produce".
The history, (and archaeology and anthropology) of Chinese agriculture pushes very far back in time-- we've got evidence of the first rice paddies some 7700 years ago. China thus has a powerful agricultural innovation in the Neolithic, a huge amount of land over which this technology can be applied, and over the next thousands of years this technology improves and more land is brought under cultivation. As of 1995, China was producing roughly 190 million tons of rice, on roughly 31 million hectares of land-- that's a yield per hectare of roughly 6 tons. While ancient yields wouldn't have been that high, we've got plenty of evidence that ancient and historical China could produce more food by bringing land under cultivation, and improving the methods in areas already in cultivation.
There is a substantial literature on the prehistory and history of Chinese agriculture, much of it from domains other than history. Archaeologists, anthropologists, ecologists, geographers, agronomists, biologists all have contributions for the curious reader.
Sources:
Zong, Y., Chen, Z., Innes, J. et al. Fire and flood management of coastal swamp enabled first rice paddy cultivation in east China. Nature 449, 459–462 (2007). https://doi.org/10.1038/nature06135
Talhelm, Thomas and Oishi, Shigehiro, How Rice Farming Shaped Culture in Southern China (June 20, 2018). Talhelm, T., & Oishi, S. (2018). How Rice Farming Shaped Culture in Southern China. In A. K. Uskul & S. Oishi (Eds.), Socioeconomic Environment and Human Psychology (pp. 53–76). New York, NY: Oxford University Press.. Available at SSRN: https://ssrn.com/abstract=3199657
Khush, G. S. (1997). "Origin, dispersal, cultivation and variation of rice". Plant Molecular Biology 35, 25-34.
Zhuang, Yijie. “Rice Fields, Water Management and Agricultural Development in the Prehistoric Lake Taihu Region and the Ningshao Plain.” Water Societies and Technologies from the Past and Present, edited by Yijie Zhuang and Mark Altaweel, UCL Press, London, 2018, pp. 87–108.
Bray, Francesca. 1984. Science and Civilisation in China. Volume 6: Biology and Biological Technology. Part II: Agriculture. Cambridge: Cambridge University Press.
To add to what /u/amp1212 has said, it is also important not to conflate the historical bounds of China with the current territory of the People's Republic. Of the roughly 9.8 million square kilometres of land encompassed by the PRC, about 6 million are the result of the Qing conquests, and consist mostly of regions of relatively poor fertility owing to severe aridity: northeast Manchuria, Inner Mongolia, East Turkestan and Tibet are broadly speaking dry and non-arable. In addition, China's southern regions, such as Guangdong, Guangxi, Guizhou and Yunnan, are largely mountainous and provide limited farmland, but these too were regions that entered the Chinese sphere of influence relatively late in the grand scheme of things, with some of these regions first coming under Chinese rule during the Han Dynasty. As such, the original 'core' of 'Chinese civilisation' on the Yangtze and Yellow Rivers was in fact located in almost completely arable land.