how can we set no of mappers and reducers for particular job?

[#1]

In java code:

Job job = new Job(conf);
...
job.setNumReduceTasks(20);
job.setNumMapTasks(20);

[#2]

@pavan p

From command line you can check -Dmapreduce.job.maps, -Dmapreduce.job.reduces and -Dmapreduce.job.running.map.limit, -Dmapreduce.job.running.reduce.limit.