SampleSiteConfigExtensionForSubiste.php

<?php

/**
 * Class CalendarSiteConfigExtension
 */
class CalendarSiteConfigExtension extends DataExtension
{

    /**
     * @var array
     */
    private static $db = array(
        'EventInstancesToShow' => 'Int',
    );

    /**
     * @param FieldList $fields
     */
    public function updateCMSFields(FieldList $fields)
    {
        $fields->addFieldToTab('Root', new TabSet('Calendar', 'Calendar'));

        $fields->addFieldToTab(
            'Root.Calendar.Settings',
            NumericField::create('EventInstancesToShow')
                ->setTitle('Event Instances To Show')
        );
    }

}