Look at the ejabberd Guide,

mfoss — Mon, 04 Apr 2011 22:47:00 +0000

Look at the ejabberd Guide, it says:

ejabberd Guide «3.3.10 mod_muc» wrote:

{history_size, Size}
... define the number of history messages to keep and send to users joining the room.
The default value is 20.

After reading that, it's easy to answer your questions:

jessy.diamondman wrote:

when I connect to a MUC, I specify that I want 100 messages

You may request 1 million, but ejabberd by default stores only the last 20 messages :P

jessy.diamondman wrote:

which makes me think that muc must be scrapping or simply ignoring the other messages

Right, the room forgets the older messages in the FIFO queue of messages.

jessy.diamondman wrote:

if anyone knows hoe to configure muc to send the number of requested messages no matter what, I would greatly appreciate it.

Look at mod_muc option: history_size. Set it to 100 or higher in ejabberd.cfg

BTW, notice that the message history is kept in RAM, not disk: when the room/muc/ejabberd is stopped, the history is lost.

ejabberd - Comments for "MUC not sending clients chat history"