I happen to know from here that there is a minimum number of messages after which a Chat Room never gets deleted. Is this true and if so what's it?
Does this rule apply to the Chat Galleries as well, or are they treated differently in this regard?
The faq says: (under Will these rooms exist forever? section)
Rooms will exist indefinitely, so long as there is at least one person actively talking in the room. A room is considered worth retaining if it has more than 15 messages by at least 2 users.
This is what you are looking for - the magic number is 15, but note that just one user posting all 15 messages is not enough (for obvious reasons) - at least one other user must also take part.
This should apply to all chat rooms all across the network.
Also note that "never gets deleted" is not 100% accurate - moderator can still delete such room manually.