According to the website, that Monzo stat is two years old. monzo.com shows the current customer count - 3.8 million ie 10% of the adult population of the UK.
According to https://www.thisismoney.co.uk/money/saving/article-7647253/M... Monzo has over half the challenger bank market share
Re:N26 will be leaving the UK
by the_mitsuhiko on Tuesday February 11, 2020 @06:06PM
(#22303941)
Monzo 139.82
From the same website — N26: 428.86 mln EUR
But i think the lesson here is that if you live in the London tech bubble, it's easy to think that Monzo are huge. Many of my friends bank with Monzo, and i see plenty of Monzo cards being tapped on the readers at the tube station. But we are not representative of the country at large.
[1] https://thebanks.eu/banks-by-country/United-Kingdom