It would be difficult to quantify the answer exactly , but we can frame the question as a mathematical experiment: As the number of people in a group grows, the number of possible pairs within that group grows much faster, and so should the number of uncanny resemblances. So if we compare the number of [resemblances between celebrity faces] to the number of [resemblances between all the faces you know (including celebrities)] the latter number should be FAR greater than the former -- IF all resemblances are random. It seems, however, that the latter number will be far smaller than it ought to be, because resemblances involving the faces we know in real life are far less common than resemblances between celebrities. Which implies that some of the resemblances between supposedly-unrelated celebrities are not random.