When will humanity see mass starvation? No one can tell for sure as there are numerous factors in the play, it is hard to predict exact decade this might happen. According to UN,
Only 60 Years of Farming Left If Soil Degradation Continues.
With that being said, there are ways to decrease crops demand, for example with change of diet, and different use of crops.
Source Currently, 36% of the calories produced by the world's crops are being used for animal feed, and only 12% of those feed calories ultimately contribute to the human diet (as meat and other animal products). Additionally, human-edible calories used for biofuel production increased fourfold between the years 2000 and 2010, from 1% to 4%, representing a net reduction of available food globally. In this study, we re-examine agricultural productivity, going from using the standard definition of yield (in tonnes per hectare, or similar units) to using the number of people actually fed per hectare of cropland. We find that, given the current mix of crop uses, growing food exclusively for direct human consumption could, in principle, increase available food calories by as much as 70%, which could feed an additional 4 billion people (more than the projected 2–3 billion people arriving through population growth). Even small shifts in our allocation of crops to animal feed and biofuels could significantly increase global food availability, and could be an instrumental tool in meeting the challenges of ensuring global food security.
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It is fair to add that not all land that is used for livestock is suitable for crops, but some might be used.
In the future we can expect way more use of cultured meat, that demand way less resources (water included), compared to conventional meat. By some estimate, cultured meat production has 82–96% lower water use depending on the product compared. It is not commercially available though, it will take some time for that to come into mass use.
SourceOf course, all this data is irrelevant in case of nuclear war, or some other similar disaster.