// Placed in functions.php
<?php

function Get_Post_Number($postID){
    $temp_query = $wp_query;
    $postNumberQuery = new WP_Query('orderby=date&order=ASC&category_name=episodes&posts_per_page=-1');
    $counter = 1;
    $postCount = 0;
    if($postNumberQuery->have_posts()) :
        while ($postNumberQuery->have_posts()) : $postNumberQuery->the_post();
            if ($postID == get_the_ID()){
                $postCount = $counter;
            } else {
                $counter++;
            }
    endwhile; endif;
    wp_reset_query();
    $wp_query = $temp_query;
    return $postCount;
}

?>

//Used to call function in template
<?php $currentID = get_the_ID(); ?>
<?php $currentNumber = Get_Post_Number($currentID); ?>

<?php echo $currentNumber; ?>