How does PARALLEL_MIN_PERCENT affect parallel execution?
I believe for that configuration you will get an error like this:


  ORA-12827: insufficient parallel query slaves available


See the references at PSORG and ORAFAQ.

If parallelism is configured differently on the server it's possible for the DOP to be downgraded. In some cases it will be downgraded to execute serially if there aren't enough PX servers available. There are a few examples here for parallel_degree_policy=MANUAL:


  Since there are only 12 available PX servers this statement gets downgraded from DOP=8 to DOP=6 and gets all 12 available PX servers.
  
  Consider another query submitted at this time with DOP=8. Even though this query requires 16 PX servers it will be given none because there are no available PX servers left. This query gets downgraded to DOP=1 and runs serially.


I have experienced that when running my own code. One way to see when DOP is downgraded is through SQL monitor. Here are a few examples of the messages you can see:


  350 DOP downgrade due to adaptive DOP
  
  351 DOP downgrade due to resource manager max DOP
  
  352 DOP downgrade due to insufficient number of processes
  
  353 DOP downgrade because slaves failed to join