How many?

Micrometres

The micrometre or micrometer, also commonly known as a micron, is an SI derived unit of length equalling 1×10−6 metre; that is, one millionth of a metre (or one thousandth of a millimetre, 0.001 mm, or about 0.000039 inch). The micrometre is often used to measure the thickness or diameter of microscopic objects, such as microorganisms and colloidal particles.

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