10-18 element aerials are used in areas with a strong signal reception.
This type of aerial, commonly referred to as a "yagi", has a reflector at the back, a main arm and a number of smaller arms or "elements". The more elements an aerial has, the higher the gain, and therefore the sharper the directivity of the aerial becomes.
Whilst most aerials follow a fairly standard design template, there is a great deal of variet within that. The type of aerial you require is very much dependant upon where you live, which transmitter you intend to use and the strength of the signal coming to your aerial.