That's the problem contemplated in this article in The Atlantic. That's one way to look at the problem, to back out from some predicted future population figure and then solve for that equation.

The author's conclusion is that biotech won't solve the issue, that the only way it's possible is if we (1) raise incomes the world over (because some people aren't fed today purely because of poverty, not because of a lack of food supply) and (2) convince people to eat less resource-intensive food, of which meat is a large culprit (the higher up on the food chain you are, the more resource-intense you are as a food source, which is why we couldn't feed many people at all if we just fed them lion meat).

Here's a long and related article in the NYTimes: Beyond the Eternal Food Fight.