This isn't specific to Mahout. In Hadoop, your capacity has no bearing on how many jobs Hadoop chooses to run. The ".maximum" properties are just expressing how many tasks to run concurrently on one worker.
Hadoop will always run 1 reducer unless you tell it otherwise with mapred.reduce.tasks. I think most of the jobs don't try to set this for you and you'll have to do so yourself. The number of maps depends on the size and number of input files. I wouldn't override it, but, if you must you can do so by decreasing the maximum split size that one mapper takes.
