Can we set arbitrary number of Reduce tasks in a mapreduce job and if yes, how ?

Yes, we can set the no of reduce tasks at job level in Mapreduce. Arbitrary number of reduce tasks in a job can be setup with job.setNumReduceTasks(N);

Here N is the no of reduce tasks of our choice. Reduce tasks can be setup at site level as well with mapreduce.job.reduces configuration property in mapred-site.xml file.