One obvious answer is that the US massively overproduces food on a scale that's hard to fathom, which is part of why we make extremely inefficient biofuels (corn ethanol) out of a big chunk of it and use it for inefficient meat production.

After some conversion through various bad units of measure (bushels->pounds->calories), and assuming ~2250 calories a day average), it looks to me like current US corn production could feed somewhere around 1.6 billion people their entire yearly caloric needs, if it was actually directed entirely at feeding people as efficiently as possible.

And that is rather obviously not the only crop or source of food originating in the US.

Math sources:

https://www.nass.usda.gov/Newsroom/2022/01-12-2022.php - corn output.

https://www.washingtonpost.com/lifestyle/food/in-defense-of-... - bushel/pound/calorie conversion

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In short, I don't think it's very realistic to think that Europe is going to face a literal inability to get enough food even if the entirety of Ukraine never grew another plant again.