It is estimated that 1 to 3 million have a concussion a year. It is estimated that 0.9% of those will have symptoms beyond a year:
Postconcussion syndrome after mild traumatic brain injury in Western Greece. - PubMed - NCBI
sorry to tired to research other articles.

Assuming that the large majority of PCS are resolved after 3 years (in the 4th year) and assuming that the average yearly number of concussions are 2 million (and assuming no repeat concussions for individuals) I get an estimate of the number of people currently dealing with prolonged PCS in the United States: 54000
This amounts to 0.02% of the population. This means this is a rare disease.

I wonder, how come there aren't many international patients on this website? where do these people go? Usually on other health related forums I find many patients living overseas.

I am dealing with 3 rare syndromes. How many get the privilege. A pat on the back....