Hadoop MapReduce: Clarification on number of reducers

Question

In the MapReduce framework, one reducer is used for each key generated by the mapper.

So you would think that specifying the number of Reducers in Hadoop MapReduce would

Answer 1

one reducer is used for each key generated by the mapper

This comment is not correct. One call to the reduce() method is done for each key grouped by the grouping comparator. A reducer (task) is a process that handles zero or more calls to reduce(). The property to which you refer is talking about the number of reducer tasks.