When I first joined deadlystream you could have 50 PMs at a time which was annoying for me. Then it went up to a 100 which was better and now with this overhaul if I am reading it correct I can now have up to 5000 PMs!
It counts topics, though. Not posts. You can have a conversation of many messages, but as long as it is under the same conversation, it takes only one message slot. Or something like that.