There is a famous quote attributed to Sir Arthur Stanley Eddington. When asked how it felt to be one of the three people in the world who understood the theory of relativity, Eddington replied, “who is the third?” (the first being Einstien himself.)
Of course this story dates back to the very beginning of the discovery of special relativity, even if it is true. By now, that number has probably increased dramatically.
Of course i myself understand all advanced theoretical physics, since i am a quark 🙂