https://curiousmindmagazine.com/all-us-presidents-including-trump-are-descendants-of-the-same-english-king/
it doesn't seem to be that uncommon to have some ancestry that was part of royalty: https://www.bbc.com/news/uk-59041055
Many of us will have shared ancestors. And if someone has ancestry in Britain going back to the Middle Ages, Prof King says it's actually more likely than not they will be related to a branch of one of the royals.
Hate to break it to you, but going back only 800 years your theoretical number of ancestors outnumbers the total of human beings who EVER lived, let alone how many were alive at the time.
If one considers as a function of time t the number of a given individual's ancestors who were alive at time t, it is likely that for most individuals this function has a maximum at around 1200 AD. It was suggested in 1985 that everyone on Earth is at most 50th cousin to everyone else, based on a relatively random mating model. Simulations published in 2004 which take into account the geographical separations and less random patterns of mating in real life suggest that some populations are separated by up to a few thousand years, with a most recent common ancestor perhaps 76 generations back, though some highly remote populations may have been isolated for somewhat longer.