@ Goldberry - A thread maxes out when it reaches the end of Page 200 (2000 posts). At that time no further posts can be made on it.
Most clans start making a new thread when their current thread is a couple of pages away from that so that they may make posts on the old thread that give out the QFC of the new thread.
The maxed thread does not need to be locked, but some clans prefer to have a padlock placed on their maxed threads to make it easier to spot the new thread when doing a thread search. If you wish the old thread to be locked once you new one is made, you may use this thread to request that.