How many calories per day are prisoners given to eat?

That depends a lot on the exact prison. Because many prisons don't follow guidelines and provide less food than they are supposed to.

In theory, inmates are supposed to get between 2,400-2,700 calories a day. The actual amount might be closer to 2,000 or even below that. For a small person that might be enough to sustain themselves but bigger people will lose weight.

Unless a prisoner has money. Then they can use the commissary store to buy additional food and eat as much as they can afford.

And that's for the entire western world. Some countries are better at enforcing guidelines than others but every country has a few prisons that are run terrible.