How many?

Millimetres

The millimetre or millimeter is a unit of length in the metric system, equal to one thousandth of a metre. There are one thousand millimetres in a metre. There are ten millimetres in a centimetre. The millimetre is used for measuring thinner things. For example, wooden flooring and underlay is often measured millimetres

Inches

The inch is a unit of length in the British imperial and the United States customary systems of measurement. Since the adoption of the international yard during the 1950s and 1960s it has been based on the metric system and defined as exactly 25.4 mm. The standard size pencil that you may see in schools and offices is 7 inches long.