This is a question that can be approached to a good approximation by reasoning it out, like the classic “how many piano tuners are there in Chicago?”
Start with a round number. Four million people are born in the U.S. each year, so two million of them are female. That mean each year 2,000,000 girls turn 18 and become legal for porn. Estimate that 1 in 100 do porn. Remember, the question is not how many become porn stars, but how many ever do even one porn act. That puts the number of new faces each year at 20,000. That’s an average, obviously. Some won’t start until they are 19 or 20 or 29. Over the course of a lifetime, though, use 20,000 per year. Let’s say the average age of death is 78, so that gives us 60 cohorts of women in the total porn pool. That’s 1,200,000 American women. That’s high, since the percentage of women turning 18 in 1953 had far fewer opportunities to do porn than 18 year olds today, but if the last few years of the explosion in year means that 2 in 100 do so then the difference will be lessened. Make it an even 1,000,000 for rounding.
Porn historically has demanded new female faces constantly, with the few men who can perform on camera dominating their side of the industry. Ten women for every men is a reasonable estimate. Add in another 100,000 men total.
The same logic can be used worldwide, with less certainty. In round numbers, the world sees about 150,000,000 births per year, therefore 75,000,000 girls. If we did a straight-line extrapolation that would give 37,500,000 women who have ever done porn. But the opportunity for many of them to do porn is probably less today than in the U.S. and was much less even a few decades ago. And the population has increased much faster worldwide than in the U.S. over the past 60 years. I wouldn’t cut that number by 10, but dividing by 5 seems reasonable. That results in 7,500,000 women total, and 500,000 men.
Those numbers seem to be in the ballpark. They include some grandmothers who did a stag film a long, long time ago, and some girls whose boyfriends used a camera to film a blowjob last week and put it on the net as well as all those who passed through the revolving door at Vivid and such places. It’s certainly not precise, but it’s a good place to start.