Example of how to output the available spaces on each datetime of an event.

example.php

<?php //Please do not include the opening PHP tag if you already have one 

function tw_ee_dipslay_datetime_spaces( $EVT_ID, $event ) {
    if( $event instanceof EE_Event ) {
        $datetimes = $event->datetimes();

        echo "<ul>";
        foreach( $datetimes as $datetime ) {

            //How many spaces are available
            $spaces_available = $datetime->spaces_remaining();

            //Check for infinity and return 'unlimited'
            $spaces_available = $spaces_available == EE_INF ? 'unlimited' : $spaces_available;

            echo "<li>There are " . $spaces_available . " spaces remaining on datetime: " . $datetime->name() . "</li>";

        }
        echo "</ul>";
    }
}
add_action( 'AHEE__ticket_selector_chart__template__after_ticket_selector', 'tw_ee_dipslay_datetime_spaces', 10, 2 );

example2.php

<?php //Please do not include the opening PHP tag if you already have one 

function tw_ee_dipslay_datetime_spaces2( $event ) {
    if( $event instanceof EE_Event ) {
        $datetimes = $event->datetimes_ordered( FALSE, FALSE );

        echo "<ul>";
        foreach( $datetimes as $datetime ) {

            //How many spaces are available
            $spaces_available = $datetime->spaces_remaining();

            //Check for infinity and return 'unlimited'
            $spaces_available = $spaces_available == EE_INF ? 'unlimited' : $spaces_available;

            echo "<li>There are " . $spaces_available . " spaces remaining on datetime: " . $datetime->name() . "</li>";

        }
        echo "</ul>";
    }
}
add_action( 'AHEE__ticket_selector_chart__template__before_ticket_selector', 'tw_ee_dipslay_datetime_spaces2', 10 );