title: number of reducers for 1 task in MapReduce
body: <p>In a typical MapReduce setup(like Hadoop), how many reducer is used for 1 task, for example, counting words? My understanding of that MapReduce from Google means only 1 reducer is involved. Is that correct?</p>
  
  <p>For example, the word count will divide the input into N chunks, and N Map will be running, producing the (word,#) list. <strong>My question is, once the Map phase is done, will there be only ONE reducer instance running to compute the result? or there will be reducers running in parallel?</strong></p>