As above I have seen stuff like that, even 2 6x9s and 2 6" rounds, even seen people put 10" subs in there (stupid) but its not my car so I don't care.
If you are talking about a VT I think the factory locations are enough, 1 in each door. Then if you don't want 6x9s in the shelf you can always go some 8" subs, there is factory brackets for them that come in Monaros and some Calais I think?
You could put 4 speakers there but I don't see the point.
yeh ive already replaced the four stock speakers and i was planning to put a pair of 6x9's on the rear parcel shelf connected direct to an amp......then a mate said i'd prob be able to fit four on there so i got me thinking...