So
28 F111
20 A4
100? Mirages
I read that the fall back, should the F-111 fail and never be delivered, was 36 F-4E, 12 RF-4C and 8 KC-135. This wasn't a numbers game, it was a rare case of determining what was needed to deliver the required effect, as opposed to an arbitrary number of squadrons.
Initially the F-111 order was meant to include six RF-111, an option that was changed after that type was cancelled, to additional F-111C (modified F-111A) as attrition replacements.
There were also long planned, but unfunded needs for tankers, AEW, strategic transports, and other force enablers, from the 60s, through the 70s and 80s into the 90s.
Ironically, an A-5B acquisition in the early/mid 60s, supplemented by RA-5C, would have met Australia's requirements, been in service a decade earlier, and could even have been assembled if not built locally.
It would also have tailed nicely into a replacement in the 80s or 90s with F-15E, F/A-18D (or even block I F/A-18F), Tornado or Mirage 2000D.
Opportunity cost is a killer. Waiting for the perfect solution often results in greater cost and a contraction in numbers and capability. Expensive life extensions become a must, maintenance cost go through the roof, availability drops through the floor, and retention becomes much harder.
Holistically Australia stuffed up pretty much all of its combat aircraft acquisitions post WWII through timing, aiming too high, missing, then going too low, while trying to save money by extending service lives, cutting numbers and skipping generations.
We really needed a generation of, good enough, combat aircraft between each of those we had. Smaller numbers (fewer attrition airframes needed), continuous production, no major upgrades, just build new aircraft with the required upgrades incorporated.