The daily word I used to know was "three people make a crowd," meaning there are at least three people in the studio. But such a vague answer doesn't fit the definition of trivia, so this article will tell you exactly who's around when the male and female actors are having sex in the studio.
The answer is, six people!

Who are these six people? The answer is director (supervisor), assistant director (AD), cinematographer (V-camera), lighting technician, sound engineer, and cinematographer (Schiu Camera). Let me explain: the assistant director (AD) does odd jobs, handling mundane tasks like buying lunchboxes and drinks. They're also on standby during filming, and when the male actor needs props like lubricant or a vibrator, they're off-camera to pass them on—they're practically never idle.
Therefore, the AD is the most basic yet indispensable position in this industry. As for the director, lighting technician, and sound engineer, I don't need to explain; even if you haven't watched a film shoot, you should know what they do in the studio. What people might be more confused about is whether the cinematographer and the cinematographer's roles are different.
Yes, there's a big difference. Photography involves filming, recording the dynamics of the male and female actors' performance, which is then edited and post-processed. Photographs, on the other hand, are static images, used to capture the action of the actors' encounters, except for the cover art.

Some might think that static photography isn't necessary?
Absolutely correct. While static photography allows production companies to have beautiful stills when releasing their work, and even create sex photo albums (meaning to make the work more like a photographic representation), strictly speaking, it's okay if they're not there on set. No stills? No problem, screenshots from the film will do, just with slightly lower quality. Can't create a photo album? Damn, if the films aren't selling, making photoshoots is a waste of money!
That's why some productions use screenshots from the film as stills—simply put, to save costs. Besides, everyone else is strictly prohibited from being present unless absolutely necessary. Even if you're a manager, you can't just stand there with your hands on your hips, watching your actress perform; please wait in the side box. And those people who constantly claim to be friends with the director and wander around the set like it's their backyard watching the filming—come on, even if you have connections, that's not how you use them!
So, including the male and female actors, there are at least eight people present in the studio—that's a lot!