Short term people can survive on 0 calories a day, but only for a few weeks (or if they are heavy, a few months).
However if you have an apocalypse scenario and food is rationed, what is the minimum for long term survival? I’m assuming 1000-1500. On that most people would probably drop to about 100-120 pounds which would give them 10-13 calories per pound. Assuming an increase in RMR efficiency due to the weight loss people could probably survive indefinitely on that, I assume.