Bill S.45 became public law No. 109-56 on 8-02-2005. As of then all certified doctors can prescribe up to his/her individual patient limit of 30, regardless to whether they are in single or group practice.
What is the 30-Patient Limit?
Senate Bill S.45 http://www.congress.org/congressorg/issues/bills/?billnum= S.45&congress=109&size=full