Mandatory Image

Hi,
you can do that either with the Custom Fields extension https://wpadverts.com/extensions/custom-fields or if you do not have access to it using the adverts_form_load filter.

add_filter( "adverts_form_load", function( $form ) {
    if( $form["name"] != "advert" ) {
        return $form;
    } 
    foreach( $form["field"] as $k => $field ) {
        if( $field["name"] == "gallery" ) {
            $form["field"][$k]["validator"][] = array(
                "name" => "upload_limit",
                "params" => array( "min" => 1, "max" => 10 )
            ); 
        }
    }
    return $form;
} );

The above code you can add in your theme functions.php file or even better create a new blank WP plugin and paste it there https://wpadverts.com/blog/how-to-use-code-snippets-in-wordpress/