How many?

Nanometres

The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American spelling) is a unit of length in the metric system, equal to one billionth (short scale) of a metre (0.000000001 m). To demonstrate just how small a nanometre is, it is commonly used to measure atoms and molecules. A typical virus is around 100 nanometres wide. The coronavirus, known as COVID 19 is believed to be between 60 to 120 nanometres wide.

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