Thanks, that’s it. Also found some extra info here:
http://modxcms.com/forums/index.php?topic=23478.0
The important thing is also to add level more than 1 (mandatory) for placeholder to work and to hide submenus (you may need this in many cases, but optional). So in other words:
&level=`2` (or more, doesn’t work with `1`)
&showSubDocCount=`1` (enables placeholder)
&hideSubMenus=`1` (hides submenus if required, i needed this as well)
after doing that, [+wf.subitemcount+] works perfectly!