If you’re not familiar with the way to specify a test on members of a hierarchy (or level), read first the page about [members].
The assertion consists in a validation of the members’ count, in the hierarchy or level specified. To achieve this, you need to specify an assertion of type count, as described here under.
Then you need to specify one of the following option:
It’s also possible to combine more-than and less-than, as shown here under.
If you’re validating that the hierarchy named “MyHierarchy” has more than 15 members: