How many?

Hands

The hand is a unit of measurement of length standardized to 4 in (101.6 mm). It is used to measure the height of horses in many English-speaking countries, including Australia, Canada, the Republic of Ireland, the United Kingdom, and the United States. It was originally based on the breadth of a human hand. The average shire horse is 16 to 17 hands tall and can even reach up to 18 hands.

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