Getting all the calories you need from (plain white) rice just about meet minimum protein needs for a sedentary lifestyle (around 50g protein). For every 100 calories of rice there are about 2.1g of protein, so for a 2000 calorie diet of just rice that would be 42g protein. But eating 10 cups of rice is a lot.

Protein-wise, an all cabbage diet would give you more if you’re meeting calorie needs - 5.1g protein per 100 calories, or 102g protein for 2000 calories worth of cabbage.. but that is a heck of a lot of cabbage (17ish lbs)!

Let’s be real though, people should be eating a varied diet and not just a single food. And perhaps not a junk food only diet.