This depends on how you define PR. Historically, PR was lumped in with RA for many, many years before it was separated into it's own diagnosis. However, many physicians still believe PR and RA are the same condition. So when you see statistics on prevalence, you have to determine if they're counting pure PR or PR lumped in with early stages RA.
According to the most recent studies, PR prevalence is somewhere between 1/20 or 1/8 of RA, which would put the number of those afflicted with PR in the USA between 65,000 and 162,000. There's also studies that suggest some people with RA start with PR and estimate that percentage around 10-15%, which would make the total afflicted in the USA around 195,000.
Due to misdiagnosis, under reporting, or simple lack of disease awareness, there are likely many more people afflicted around the world. But it's generally believed that PR is fairly rare, which is confirmed by the estimated number of .06% of the US population.
As for age and gender, it's known to affect men and women equally and is typically diagnosed between 20-50.