How does PARALLEL_MIN_PERCENT affect parallel execution? From the reference for PARALLEL_MIN_PERCENT:

PARALLEL_MIN_PERCENT lets you specify the minimum percentage of the requested number of parallel execution processes required for parallel execution. Setting this parameter ensures that parallel operations will not execute unless adequate resources are available. The default value of 0 means that no minimum percentage of processes has been set.
Consider the following settings:
PARALLEL_MIN_PERCENT = 50
PARALLEL_MIN_SERVERS = 5
PARALLEL_MAX_SERVERS = 10
If 8 of the 10 parallel execution processes are busy, only 2 processes are available. If you then request a query with a degree of parallelism of 8, the minimum 50% will not be met.

From Initializing and Tuning Parameters for Parallel Execution:

[PARALLEL_MIN_SERVERS] Specifies the minimum percentage of requested parallel execution processes required for parallel execution. With the default value of 0, a parallel statement executes serial if no parallel server processes are available.

What happens when the minimum 50% of servers is not available for the query when it runs? Is the query is queued, does it execute serially, or does it throw an error such as insufficient parallel query slaves available?
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