The original Godzilla was killed by a weapon called the oxygen destroyer.I highly doubt that Godzilla is one being if there's 73 movies done with 'this actor'.
Over how many years?
I am no expert on these beings and how old they can become whatsoever though.
But from speculations, it might sound like it's a lineage of partly big strong female apes, and we might also assume that there's some possibly larger male Godzilla's somewhere along the lines too, if they share something with how smaller monkeys procreate themselves.
Every subsequent Godzilla has been a different animal--sometime reptilian, sometime amphibious. My guess is there's a hotbed of mutant animals that fight it out Highlander style. That would explain why only one Godzilla appears at a time.