"enough light" is a relative term. It will depend on how close the light is, and what ISO and aperture you are willing to use.
Additionally, one light may provide enough light to illuminate the group, but may cause shadows due to placement. This is not an issue of the power of the light or efficiency of the modifier, and may require a second light.
If you place a light directly in in front of the group, then you will not get deep shadows, but you will have very flat lighting.
Anther thing to consider is the size of the group. A small school may have just enough players to make a team, while others will have many more players. The more people, the deeper and wider the group is. Obviously, left and right coverage is a concern. However, so is the depth. Without going into the inverse square law fully, subjects closer to the light will be brighter than those farther away. Backing off the light will even the out light front to back, but it will leave you with less light.
The Soft silver PLM will give you a 10-12 foot coverage at a 14' distance, and it will give you F/16 at ISO 100.
At 20', you would have f/11, and about 15' coverage.
I think you could get by with one light and one PLM, unless you had a large group. Here is a link to a review of our PLM's by someone who does team sports photos.
http://www.robgalbraith.com/multi_pageb ... 0046-10396