Example of how to alter the MER cart model success message.

example.php

<?php

//Alter the successfully added for this event string
function tw_ee_change_cart_result_message($original_text, $ticket_count) {
    return sprintf(
        _n(
            '1 item was successfully added for this event.',
            '%1$s items were successfully added for this event.',
            $ticket_count,
            'event_espresso'
        ),
        $ticket_count
    );
}
add_filter('FHEE__EED_Multi_Event_Registration__get_cart_results_results_message', 'tw_ee_change_cart_result_message', 10, 2);