A countertop microwave does not require a dedicated circuit.
However it's best for the countertop microwave to have a dedicated circuit because the microwave uses quite a bit of power.
A microwave could dim lights or trip breakers if other appliances are on the same circuit.
I have a dedicated circuit in my kitchen for my microwave which is much better.
A countertop microwave should easily be able to run on a 15 amp circuit.
I lived in a mobile home that had 15 amp electrical circuits in the kitchen and I ran my countertop microwave on the 15 amp circuit just fine.
Although having a 20 amp electrical circuit would be much better but a 15 amp electrical circuit will be good enough to handle the microwave.
But if possible it's best to have a dedicated 20 amp circuit to handle the power of the microwave without dimming lights.
I now live in a regular home that has 20 amp circuits and a dedicated 20 amp electrical outlet in the kitchen for the microwave.
It's much better that way but you can run the microwave on the non dedicated circuit whether a 15 amp circuit or 20 amp circuit.