As a whole, the Combine is a vast Galactic Empire. In HL2, Dr. Breen served as Leader of Earth, but was still under the control of The Advisors, so it's very likely that on the many other worlds they control, there is a selected leader being governed by Advisors. Advisors themselves are thought to be a type of Council that control The Combine, but their name also suggests that there could be a Supreme Leader, such as an Emperor. This however, is currently unknown.