You can easily set the number of 'reducers' just by changing few parameters in your program.
One way to achieve this is by updating the driver program and set the setNumReduceTasks to the desired value on the job object.
Alternatively, you can set below property as a command line parameter by using -D option
For example you can use it like as mentioned in the below command:
hadoop jar hadoop-mapreduce-examples.jar wordcount /InputDir /OutputDir