Force list widget to show 15 events

add_filter( 'widget_display_callback', 'force_list_widget_to_show_15', 10, 2 );

function force_list_widget_to_show_15( $instance, $widget ) {
    if ( 'tribe-events-adv-list-widget' !== $widget->id_base ) return $instance;
    $instance['limit'] = 15;
    return $instance;
}