From the doctrine of traditions, the more the number of participants in congregational prayers the more rewards are obtainable until such time that there are ten participants, in which the rewards become so great that it is immeasurable, and no one other than God will know its true value.
Consider what one can do with their fingers. A person can with one finger dial a number on the dial pad of a telephone, and with two fingers lift a small bucket, with three fingers do slightly more work, until you reach a point where the work that can be done with ten fingers is limitless. Any work that one wishes to do can be done with those ten fingers, leaving the number of applications for those ten fingers to be boundless.