The minimum amount of food that can be placed in a microwave oven at one time can vary depending on the size and specifications of the microwave. However, in general, you can safely place even a small quantity of food in a microwave without any issues. Most microwave ovens are designed to heat food evenly, regardless of the amount.
For instance, you can microwave a single slice of pizza, a small cup of soup, or even a small portion of leftovers. Just make sure to use microwave-safe containers and follow any guidelines provided in the microwave's user manual.
Keep in mind that the cooking time might need adjustment for smaller quantities, as cooking times in a microwave are often based on the volume and density of the food being heated. It's also essential to use a microwave-safe dish and cover the food with a microwave-safe lid or microwave-safe microwave cover to avoid splatters and spills.