The last time I did a deep dive on the research, they estimated somewhere around 3% of the population had some form of chimerism, and I calculated my personal chances around 6%. And then I did some family research and anecdotal evidence pushed that number much higher, including being a single born twin.

One of the articles I recall postulated the number is much higher than 3% due to the condition only being confirmed or discovered through rare circumstances that result in multiple genetic testing.