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.

Microinches

A microinch is a unit of measurement, the length of one one-millionth of an inch, equivalent to 25.4 nanometers. Being such a small measurement, micorinches are often used in precision machining and to measure surface finishes.