They do not know even vaguely how rare the condition is, because most people who have it don't realize that everybody else can see faces so much better, and the condition is so rare that the general public is unaware of it as being a possibility. Thus many people never come in to be tested, diagnosed, or whatever. Nevertheless, they have social and employment problems. Much of the literature is about people who got it due to blows to the head. Little is about those of us who were born with it, though they do say it very much runs in families when that is the case.
Some people have a mild case of it.
Others can not identify any faces at all. One man could not for sure even identify his own face. There appears to be a lot of work left for us to do. The literature goes no further than to say we rely on clothes, hairstyles, beards, gait, and voice. Which occupations work best for us, and which ones are real bummers.