I tested it, if you try to place two hud messages in example channel 2, #2 hud message will choose anyohter available channel or if there are no available channels then one of hud message will be gone.
Tested Dhud, i can show much more than 4.
There is a one way to show normal hud more than 4, but you need to recreate the hud message system in server side. It will be little bit hard