Custom menu widget – how to display number of posts?

I’d like my custom menu widget to display the number of posts next to the category, like the category widget does.

Site: https://www.planestrainsandplantagenets.com/

Currently it just has the category names, ie:

London
England
Scotland

I’d like it to say

London (7)
England (3)
Scotland (4)

and so on.