If you are looking for tenets of the COIN religion, you can never go wrong Googling John Nagl (Peace Be Upon Him).

The first requirement for success in any counterinsurgency campaign is population security. This requires boots on the ground and plenty of them—20 to 25 counterinsurgents for every 1,000 people is the historically derived approximate ratio required for success, according to the U.S. Army/Marine Corps Counterinsurgency Field Manual. That force ratio prescribes some 600,000 counterinsurgents to protect Afghanistan, a country larger and more populous than Iraq—some three times as large as the current international and Afghan force. The planned surge of 30,000 additional American forces to Afghanistan over the next year is merely a down payment on the vastly expanded force needed to protect all 30 million Afghan people.
- via US News

Just be advised that the ratio is misguided at best. That historically derived approximate ratio has as it's input data Soldiers who formerly did not have the ability to call in Spectre, F-16's, or Apaches, benefit from the extra set of eyes from Kiowas and UAVs, communicate securely with individuals 50 meters away or 50 km away instantly, or rely on world-class medical care where you get from the battlefield to the operating table in an hour. Soldiers included in that historically derived approximation also include guys who carried rifles more appropriate for hunting than combat, who did not need to worry about the media looking over their shoulders and cheap-shotting them, and on, and on.

In something as complicated as war, ratios like that are not very useful.