title: The parameter of "Flink on YARN" when use "Run a single Flink job on YARN " model
body: <p>if i want to run a flink job on yarn ,the command is
  <code>./bin/flink run -m yarn-cluster ./examples/batch/WordCount.jar</code></p>
  <p>but the command will run a default cluster which have 2 taskmanagers ;
  if i am only submmit single job,why the default taskmanagers is setted  2?</p>
  <p>and when do I need mutiple taskmanager in single job?</p>