> How many of those backdoors are still to be discovered?

Since keeping such backdoor hidden in plain sight is extremely hard and required tons of preparation and social engineering spanning multiple projects, the answer is probably a function of number of those already discovered. As we don't discover years-old similar backdoors every now and then and had discovered this one pretty quickly, this might very well be the very first one that came this far.

Also, what's "sheer chance" for an individual is "enough eyeballs" for a collectivity.