Its been standard practice at YMMB for about Ten years as far as I can tell.
I agree with KingRB it is for the controller (maybe CAGRS as well?) to designate your number in a circuit and may be lethally stupid to try and do it yourself and announce it to the world....or not.
I also don’t recall a controller at YMMB going past number three. I guess it is a bit academic after that and most pilots can't count past Five anyway unless in bare feet.
My personal limit at Moorabbin if I'm out doing circuits is about Six, depending on the quality of the radio transmissions I'm receiving and the behaviour of everyone else. When I hear in broken English of an imminent arrival from Sandown plus another at Parkmore, both requesting circuits on arrival, I make the next one a full stop.