completely impossible to determine that number. If anyone could even think about finding it,
@cerich would. You could probably find out which ones have sold the most, but then you have to look at subdividing by model numbers. I.e. the Scubapro 109/156/G250/G250H/G260 are all essentially the same regulator, but all different model numbers. The big issue is you have no idea which ones have been trashed/scrapped etc. and no way to track that from the manufacturer. Very different than vehicles that are registered.
If I had to place a guess, and I wouldn't bet on it, but if someone asked me and I had to guess, I would guess the Poseidon Cyklon, and only because it has been in continuous production for 60 years and is the only regulator that has anywhere near that longevity. On age alone I would bet there are more of them produced than any other regulator. Scubapro probably could have them beat if they didn't change their model numbers ever 5 years, i.e. I'd bet there are more 109/156/g250/g260's out there than Cyklons, but it depends on how you qualify your search. If it's by brand, Scubapro, if it's by family, Scubapro, if it's a specific model, probably Cyklon.