If you want to limit the recent posts on the board index this is how to do it.
In Recent.php find
$request = db_query("
SELECT
m.posterTime, m.subject, m.ID_TOPIC, m.ID_MEMBER, m.ID_MSG,
IFNULL(mem.realName, m.posterName) AS posterName, t.ID_BOARD, b.name AS bName,
LEFT(m.body, 384) AS body, m.smileysEnabled
FROM ({$db_prefix}messages AS m, {$db_prefix}topics AS t, {$db_prefix}boards AS b)
LEFT JOIN {$db_prefix}members AS mem ON (mem.ID_MEMBER = m.ID_MEMBER)
WHERE m.ID_MSG >= " . max(0, $modSettings['maxMsgID'] - 20 * $showlatestcount) . "
AND t.ID_TOPIC = m.ID_TOPIC
AND b.ID_BOARD = t.ID_BOARD" . (!empty($modSettings['recycle_enable']) && $modSettings['recycle_board'] > 0 ? "
AND b.ID_BOARD != $modSettings[recycle_board]" : '') . "
AND $user_info[query_see_board]
ORDER BY m.ID_MSG DESC
LIMIT $showlatestcount", __FILE__, __LINE__);
Replace with
// Board to show
$boards_to_show = array(6,7,16);
$request = db_query("
SELECT
m.posterTime, m.subject, m.ID_TOPIC, m.ID_MEMBER, m.ID_MSG,
IFNULL(mem.realName, m.posterName) AS posterName, t.ID_BOARD, b.name AS bName,
LEFT(m.body, 384) AS body, m.smileysEnabled
FROM ({$db_prefix}messages AS m, {$db_prefix}topics AS t, {$db_prefix}boards AS b)
LEFT JOIN {$db_prefix}members AS mem ON (mem.ID_MEMBER = m.ID_MEMBER)
WHERE m.ID_MSG >= " . max(0, $modSettings['maxMsgID'] - 20 * $showlatestcount) . "
AND t.ID_TOPIC = m.ID_TOPIC
AND b.ID_BOARD = t.ID_BOARD" . (!empty($modSettings['recycle_enable']) && $modSettings['recycle_board'] > 0 ? "
AND b.ID_BOARD != $modSettings[recycle_board]" : '') . "
AND (FIND_IN_SET(" . implode(', t.ID_BOARD) OR FIND_IN_SET(', $boards_to_show) . ", t.ID_BOARD))
AND $user_info[query_see_board]
ORDER BY m.ID_MSG DESC
LIMIT $showlatestcount", __FILE__, __LINE__);
Change $boards_to_show to the id of the boards that you want to show. Separate each id with a ,