There is a limit variable built into the function calling the data that is unused in the default install.
On line 26 of Logs.php you will find
$this->set('all_logs', LogData::findLogs($params));
If you look in LogsData.class.php on line 22 you will find
public static function findLogs($params, $count = '', $start = '')
the $count and $start variables are there to add pagination if you would like or you can use just the $count variable to limit the number of lines returned from the database.
To achieve a limit of data returned you could add the $count variable to your data call in Logs.php line 26 like
$this->set('all_logs', LogData::findLogs($params, '20'));