In on-page edit or in properties mode?
It should be fairly easy to add a tinymce plugin for this, lots of these plugins can be found on the web. And the official docs regarding how to add a custom tiny mce plugin can be found here: https://world.episerver.com/documentation/Items/Developers-Guide/Episerver-CMS/9/Editing/Customizing-the-TinyMCE-editor/#addplugin
Hi Krzysztof this is probably what you are looking for:
David, you made my day! Thank's a lot
Hello, our editors want to have characters counter displayed under textarea in edit mode. When it comes to dojo I'm completely new. I'd really appreciate some help with this...