What do you think the count should be?

MW requires a page to have a link to be counted as an article

https://www.mediawiki.org/wiki/Manual:Article_count

Do also note, a script is run at least monthly to recalculate the number of articles, so there can be weird jumps occasionally depending on changes done to a page

Aha, a link is needed. There indeed are some articles without any link (example).

@Janezdrilc: Does that mean that this page is invalid, as the software behaves as documented?

Still not sure ...

Links definitely seem to be a problem. There had been few hundred articles without any link. Adding Infobox Person which consists of internal and external links has made many of them to be recognized for the first time. But, some infoboxes include just external links, so I saved an internal one into the infobox template (just for test). Surprisingly, article count was unchanged. At least this article should make a change in number since his only internal link is a "testing link" of the infobox.

There are still about 160 "empty" articles waiting for linking (Special:DeadendPages). Adding links to 3 of them has made article count immediately updating the number. That works as expected.

Isn't this similar with T212706?