Electrician put in a new breaker for the microwave.
Only the microwave is on the circuit. But the Microwave
Operates about 30 times okay. But then the new breaker trips
Off and has to be reset. Is it the microwave?

Happy new year Pastor.

You have left out some critical information:

1) What is the amperage of the new breaker?
2) Did the electrician run a new circuit to the microwave?
3) If the electrican put a 20 amp breaker is for the microwave circuit, did he/she run
12 AWG conductors to the receptacle outlet box?
4) What is the amperage required by the microwave?

A microwave's motor just like all other motors has a large in-rush current. If this in-rush current is high is close to the trip current of the breaker that could be your problem.

It is best to put a microwave on a 20 amp circuit because of this.

I have to look. I know it has a 20 amp breaker. He didn't rewire. While he was there
It put it on and off above 5 times. Worked okay. But on the start up at the 20th time.
It tripped the breaker. Maybe rush-in current?

You say, "he didn't rewire", you can be sure there are other things on this circuit.
If this tripping breaker is inconvenient, have the electrician install a new wire with a new circuit breaker just for the microwave...
Microwaves should have a dedicated circuit.
If not, only run the microwave 19 times a day...

We will have to see, buying a new microwave anyway. Inside rusted out.
Thanks Again