How do I ensure that there's only reducer for my EMR Streaming job? Is there any way to do this from the web frontend when I'm creating a new Jobflow?
You can configure Hadoop in the bootstrap action using the --arg flag. Specifically to your question you can set the mapred.tasktracker.reduce.tasks.maximum to 1.
elastic-mapreduce --create --alive \
--name "Configure Jobflow" \
--bootstrap-actions s3://elasticmapreduce/bootstrap-actions/configure-hadoop
--arg mapred.tasktracker.reduce.tasks.maximum=2