Magento2 Display dynamic data in XML

I have created a custom Tab on the product details page. Now I want to show the count of those articles, like how Reviews count are shown in tab title.

<arguments>
     <argument translate="true" name="title" xsi:type="string">Frequently Asked Questions</argument>
</arguments>     

This is the code used to show the tab. Now how can I show the count.

I have tried to use the Class in my block class like -

use \Magento\Framework\DataObject\IdentityInterface as IdentityInterface;
use \Magento\Framework\View\Element\Template as Template;
class Customtab extends Template implements IdentityInterface

and then -

public function setTabTitle()
    {
        $title = 'Frequently Asked Questions';
        $this->setTitle($title);
    }

But it is giving -

Fatal error: Class Vendor\Module\Block\Product\View\Details\Customtab contains 1 abstract method and must therefore be declared abstract or implement the remaining methods