Hurtado, Borja, and Rodriguez are still alive (all pre-1992), giving Ecuador something like 12 living heads of state still alive.
And of course in the Commonwealth realms, there are 0 living former heads of state.
Australia has 5 living Prime Ministers (Rudd, Guillard, Howard, Keating & Hawke) having lost two (Fraser & Whitlam) in the past year.
There are 6 living Governor Generals (Bryce, Jeffery, Hollingsworth, Deane, Hayden & Stephen) though, as indicated, there are no living ex Heads of State.
Acting Prime Ministers are rather more common, as they are delegated when the Prime Minister is either out of the country or is on holiday so there are at least 9 of them (Albanese, Swan, Gillard, Vaile, Anderson, Fischer, Beazley, Howe, Keating). My google fu has been unable to determine whether any 3rd in line have been Acting PM when both the PM and Deputy PM were off duty.
So as either PM or Acting PM there are 12 of them, but that’s too long a bow.
I’d leave the current count at 5.
Man, I was all set to be pedantic here, but then I noticed that you said “Commonwealth realms”. There are of course Commonwealth republics (and even a few Commonwealth monarchies), for whom Elizabeth II is not the head of state; but by definition she is head of state of all the realms.
Unless you take the view expounded by the Australian Monarchist League, which is that the head of state of a Commonwealth realm is the Governor-General.