A single-entity charter is one in which an individual or company charters a plane and bears the entire cost of the flight, so that the passengers do not pay their own airfare. There is no minimum passenger requirement, since the cost is per flight, not per person. Single-entity charters are typically used for business purposes -- for example, travel to meetings and conferences, incentive travel or VIP leisure travel.
Though the early Lockheed Jetstar had four, most production business jets have two jet engines, mostly rear-mounted podded engine. If mounted below their low wing, it wouldn't allow sufficient engine clearance without a too long landing gear. The HondaJet is the exception with its over the wing engine pods. Dassault Falcon still builds three-engine models derived from the Falcon 50, and the very light jet market has seen several single-engine design concepts and the introduction of the Cirrus Vision SF50 in 2016.