Q: How much oil needs to leak into an Alternator to cause the Alternator to go bad

I sent my car with my Mechanic Friday afternoon to fix brakes, change fan and computer. My mechanic said car would be done and returned Saturday morning. Saturday afternoon, he calls to tell me that I need a new battery and alternator. He said he drove it made several stops and Saturday morning took it to pick up parts. Upon returning to my car it would not start. He removed the battery took it home to charge and found battery and alternator needed to be replaced. Alternator had oil leakage in it. He replaced battery and alternator made several stops again and dropped my car off Sunday Evening. My husband drove the car Monday morning to drop my son off at work and upon returning home battery lights came on radio died and car died. He had to push car into parking stall. I called the mechanic and he came to diagnose the problem. Took the alternator, battery and its been a week. He now comes and says I now need a new battery, alternator and gasket valve.

My car has 100000 miles.
My car's transmission is unknown to me.