I don't think anyone can tell you just how widespread losses of clams are, because for everyone that posts about a loss there are many more who have lost clams who don't post.
Incidentally, it is safe to say that those who have gorgeous pictures of many clams in a tank have recently acquired their clams, and their system has not been sustaining clams for years.
I am still waiting for someone to prove me wrong. And, specimens of gigas, derasa, and squamosa do not count. These have been proven to be sustainable in aquaria for years with no problems.
If you are concerned about disease, I would resist the temptation to overstock a tank with clams. You can keep large numbers of clams in a system but they require special care. Get one or two clams for a moderate sized reef, and try to buy them locally if possible. Or better still, buy them from someone who is knowledgable about clams.
It is up to you to talk to whoever it is you will potentially buy clams from and match what they say with what is written in the literature. Barry at clamsdirect seems to do a lot of business with reefers here, but his clams are expensive IMO. You could also try Dr. Mac Corals. He seems to be knowledgable as well, and I would trust the observations of a licensed DVM any day.
Just some food for clam thought.