It doesn't necessarily need to have a dedicated circuit but, if it's shared with other loads, it can't exceed 50% of the circuit rating. 210.23
Now that I agree with - and in a lot of cases this would push towards a dedicated microwave circuit. There are still cases (such as a microwave and small disposal) where they can be on the same circuit.
This is common when planning kitchens, that you need the manufacturers instructions and appliance ratings to plan the branch circuits for microwave, disposal, dishwasher, range hood, etc.