1500 for the average woman, 2000 for the average man. For perspective, fidgety people burn about 100 calories more than calmer. Recrossing your legs, drumming on on your thighs, and picking at your teeth with your tongue uses 100 calories. Walking, getting a new ream of printer paper, the "oh poop, the train" shuffle, thinking about your ex and how they are a narcissist, and keeping yourself warm when the wind kicks up all take about the numbers I said, 1500 or 2000 depending on sex.
There's a number of systems that take over and then fail as we starve but stay alive but to avoid them, many of which are called diets these days, the biggest tell is blood sugar. The biggest tell of low blood sugar and that you're not getting enough calories right now is hunger and mood, aka "hangry." But the simple want of food is a perfect sign, better than an app that says you need more calories to avoid backup system activation and ultimate failure.