No , Canada is purchasing upto 25 for replacements and spares. I believe a private air training firm in the US is buying more RAAF Hornets, but the exact number AFAIK has not been determined.
I have to ask though, what advantage would there be for the RAAF to purchase more F/A-18 Super Hornets? Right now there are essentially only three variants available, the single seat -E, the twin seater -F currently in RAAF service, and the SEAD/DEAD version the EA-18G Growler, also in RAAF service.
With the F-35A being a newer, and more all around capable fighter aircraft than the SHornet, which at this point has been an in-service design for 20 years now, there is really only one advantage to operating a mixed fleet of fighters. I am purposefully not referring to the special abilities of the Growler, simply because Australia already has them in service. The sole advantage I see in operating a mixed fleet would be if something catastrophic were to happen which caused a grounding of all Australian F-35A's, then the RAAF would have a second type of fighter which it could press into service while the F-35A grounding issue was resolved.
Otherwise, in terms of aircraft capabilities and service outputs, I see no reason to add any additional units to the RAAF inventory. The one possible exception would be to purchase (or possibly convert) a replacement for the EA-18G which was lost.