Docusaurus didn't provide an easy way to retrieve the number of blog posts but there is well a trick.
There is an automatic page called
archive like https://www.avonture.be/blog/archive/.
On that page, all blog posts are displayed by year and months. With a
document.querySelectorAll console instruction it's possible to make the count as suggested on https://github.com/facebook/docusaurus/discussions/9712
Once the archive page has been displayed, press F12 to start the Developer console.
Click on the
Console tab then, in the right bottom part, copy/paste the instruction below.
document.querySelectorAll("#__docusaurus_skipToContent_fallback > main > section > div > div > div > ul > li")
The first time, the console will notify you that you should allow pasting (in French autoriser le collage). Please type it.
Then copy/paste again.
document.querySelectorAll instruction will retrieve every bullet i.e. the list of posts. You'll immediately see the number, in my case, as illustrated below,