function get_post_number( $previous = false, $same_term = false, $taxonomy = 'category' ) {
    global $wpdb;

    if ( ( ! $post = get_post() ) || ! taxonomy_exists( $taxonomy ) )
        return null;

    $current_post_date = $post->post_date;
    $join = '';
    $where = '';

    if ( $same_term ) {
        $join .= " INNER JOIN $wpdb->term_relationships AS tr ON p.ID = tr.object_id INNER JOIN $wpdb->term_taxonomy tt ON tr.term_taxonomy_id = tt.term_taxonomy_id";
        $where .= $wpdb->prepare( "AND tt.taxonomy = %s", $taxonomy );

        if ( ! is_object_in_taxonomy( $post->post_type, $taxonomy ) )
            return null;

        $terms = get_the_terms( $post->ID, $taxonomy );
        if ( $terms ) {
            $terms = wp_list_sort( $terms, array( 'term_id' => 'ASC' ) );
            $term = $terms[0];
            $where .= " AND tt.term_id = {$term->term_id}";
        }
    }

    if ( is_user_logged_in() ) {
        $user_id = get_current_user_id();

        $post_type_object = get_post_type_object( $post->post_type );
        if ( empty( $post_type_object ) ) {
            $post_type_cap    = $post->post_type;
            $read_private_cap = 'read_private_' . $post_type_cap . 's';
        } else {
            $read_private_cap = $post_type_object->cap->read_private_posts;
        }

        $private_states = get_post_stati( array( 'private' => true ) );
        $where .= " AND ( p.post_status = 'publish'";
        foreach ( (array) $private_states as $state ) {
            if ( current_user_can( $read_private_cap ) ) {
                $where .= $wpdb->prepare( " OR p.post_status = %s", $state );
            } else {
                $where .= $wpdb->prepare( " OR (p.post_author = %d AND p.post_status = %s)", $user_id, $state );
            }
        }
        $where .= " )";
    } else {
        $where .= " AND p.post_status = 'publish'";
    }

    $op = $previous ? '<=' : '>=';
    $order = $previous ? 'ASC' : 'DESC';

    $where = $wpdb->prepare( "WHERE p.post_date $op %s AND p.post_type = %s $where", $current_post_date, $post->post_type );
    $sql = "SELECT COUNT(*) FROM $wpdb->posts AS p $join $where ORDER BY p.post_date $order";
    $number = (int)$wpdb->get_var( $sql );

    return $number;
}

<?php
	$chosen_id = $catid; // カテゴリID
	$thisCat = get_category($chosen_id);
?>

<?php
	$number = get_post_number( false, true, 'category' );
?>

<div class="pagination">
  <div class="prev"><?php next_post_link('%link', 'PREV', true) ?></div>
  <div class="page_num_wrap"><span class="page_num"><?php echo "$number"; ?>/<?php echo $thisCat->count; ?></span></div>
  <div class="next">
    <?php previous_post_link('%link', 'NEXT', true) ?>
  </div>
</div>