Add Counter in plugin.yaml

1. In your plugin.yaml, add counter and counterLabel like this:

side-menu-item:
  label: yourLabel
  url: your/controller/url
  icon: your-icon
  ...
  counter: \AuthorName\PluginName\Models\YourClass::Yourfunction
  counterLabel: Your Label

2. then in your model PHP file, add your function which is returning a number like this for example :

<?php namespace  \AuthorName\PluginName\Models;

class YourClass extends Model{

  ...

  public static function Yourfunction(){
    // you can do whatever you need right here, you just need to not forget to return a number.
    return self::count();

  }
}

3. check the result in your backend