I'm not sure exactly the number of messages, but I think you only reach the limit when you send too many messages in a short amount of time. It's put in place to prevent spamming. It will reset itself in a few hours, I believe.
You can only send so many messages in a period of time, if you exceed this then you will be prevented from sending any more messages until the limit goes away. This takes about an hour, and resets every time you send messages on the chatter and such.