Get number of child pages for menu styling

Hello, first post here!
I've been experimenting Grav for some weeks and I love it! Modular pages are a stunning feature so is its speed.
That said, I ended up trying to create a theme fitting my needs.
I'm not completely new to web programming however my (limited) experience has been paused for the last ten years and a lot has happened. I primarly worked with PHP + CSS 2 and a little bit of jQuery.
I decided to start with the minimal Pure CSS framework, here's the first issue: how do I output in HTML the number of pages of the menu? This is needed in order to use Pure's Grid System i nnavigation menu.
I tried children.count() but it doesn't seem to fit this case. Or maybe I'm using it the wrong way.
Thank you!
Marco

Thank you, this just works!
My aim is to build a photo gallery based off PureCSS grids and those variables will be useful.
But I'm still far from this point, I'm struggling with navigation menu 😅
I'm also trying to understand how the Docs work as I'm not that used to such schemes (coming from WordPress Codex).
Thank you!