Problem querying number of topics in a forum

Short version: the aggregate needs to be by forum, not by topic, and you must use the aggregated result keyed by forum_id when rendering each forum row. @kaykays was on the right track — grouping by forum_id returns one count per forum. The symptom you saw (same number for every forum) often comes from using mysql_num_rows on the wrong result set (that returns how many rows the query returned, not the number of topics for the currently-displayed forum) or from grouping incorrectly and/or using an INNER JOIN (which hides forums with zero topics).

A reliable pattern is to run a single aggregate query that returns forum_id => topics_count, load that into an associative array, then use that array inside your forums loop. Example (SQL then PHP):

SELECT f.forum_id, f.forum_name, COUNT(t.topic_id) AS topics_count
FROM forums AS f
LEFT JOIN topics AS t ON t.topic_forum = f.forum_id
GROUP BY f.forum_id, f.forum_name
ORDER BY f.forum_id;

$topic_counts = array();
$res = mysql_query($sql);
while ($r = mysql_fetch_assoc($res)) {
  $topic_counts[$r['forum_id']] = (int)$r['topics_count'];
}

/* when outputting each forum row:
   echo isset($topic_counts[$row_forums['forum_id']]) ? $topic_counts[$row_forums['forum_id']] : 0;
*/

Notes and tips: use COUNT(t.topic_id) with a LEFT JOIN so forums with zero topics yield 0; include the non-aggregated columns in GROUP BY to satisfy SQL modes like ONLY_FULL_GROUPBY; avoid running a separate COUNT query inside the display loop for every forum (slow); and move away from deprecated `mysql*functions to MySQLi or PDO for newer PHP versions. See the MySQL COUNT documentation and the PHP manual formysql_num_rows` for details: MySQL COUNT docs PHP mysql_num_rows.