How many countries have not officially adopted the metric system?

How to get close to this number

The answer has to be small, and knowing that is most of the work. Metric is the basis of international trade, science and engineering, so a country outside it pays a real cost. Only a handful of exceptions could survive that pressure, and the famous one is famous precisely because it is nearly alone.

The nuance in the explanation matters more than the count. Official adoption and everyday usage are different things: a country can be legally metric and still sell beer in pints and measure distance in miles, as Britain does. Conversely the country usually cited as non-metric uses metric units throughout its science, medicine and military. Legal standard and street practice diverge almost everywhere.

Which is why this figure is really about official status rather than behaviour. The source is an advocacy association, and definitions of "officially adopted" vary — some lists count countries that have legislated metrication but not completed it. Treat it as a well-known shorthand for a more complicated reality, and note that the interesting fact is the divergence rather than the number.