It takes 1000 calories of feed to make 100 calories of animal protein. While the type of animal feed is not what humans probably want to eat, the calories already exist today. Without livestock, there would be a huge surplus of available food.

Wikipedia[0] Says that US directs 4345 million bushels of just corn to livestock feed. Which would be ~13 bushels or 330kg per American.

[0] https://en.wikipedia.org/wiki/Corn_production_in_the_United_...