There is usually only one reflector and one or more directors. A yagi antenna is rated by its total number of elements. A yagi antenna may be considered as a directional form of a dipole antenna and the comments on the dipole antenna are applicable. Failure to do so may result in eventual damage to the antenna. If mounted for horizontal polarization, ensure that the antenna feeder cable entry to the dipole element is at the bottom to allow condensation to drain.
Large yagis are best center mounted rather than end mounted.
Mounting a vertically polarized antenna centrally is not recommended as the mast is also vertical and will affect the antenna performance. If end mounting a large antenna it is best to use a bracket recommended by the manufacturer to avoid possible degradation of the antenna's characteristics. Miscellaneous Turnstile antennas take two dipoles and mount them 90 degrees to each other. It is a poor attempt to get an omnidirectional antenna. Many hobby electronics stores sell these for FM antennas, and they only work because the FM signal in the city areas is usually quite strong.