You don’t need to use jQuery for something like this, it would be better done if you just use php!
So if you store those 3 values in variables you can then do basic math with that data about output the results.
while ($query->have_posts()) :
$query->the_post();
$tourNum1 = get_post_meta($post->ID, 'tour_details_number', true);
$tourNum2 = get_post_meta($post->ID, 'tour_details_number2', true);
$tourNum3 = get_post_meta($post->ID, 'tour_details_number3', true);
$total = 20 - ($tourNum1 + $tourNum2 + $tourNum3);
echo '<li>';
echo '<div>';
echo '<span class="num1">';
echo $tourNum1;
echo '</span>';
echo '<span class="num2">';
echo $tourNum2;
echo '</span>';
echo '<span class="num3">';
echo $tourNum3;
echo '</span></div>';
echo '<a href="https://wordpress.stackexchange.com/reserve-items/?add-reserve=";
echo the_id();
echo "">';
echo the_title();
echo '</a>';
echo " <span class="count">".$total."</span></li>";
endwhile;