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.

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.