Think of all the faces you know. As you flick through your mental Rolodex, your friends, family, and co-workers probably come first—along with celebrities—followed by the faces of the nameless strangers you encounter during your daily routine.

But how many faces can the human Rolodex store?

To ballpark the size of the average person’s “facial vocabulary,” researchers gave 25 people 1 hour to list as many faces from their personal lives as possible, and then another hour to do the same with famous faces, like those of actors, politicians, and musicians.

If the participants couldn’t remember a person’s name, but could imagine their face, they used a descriptive phrase like “the high school janitor,” or “the actress from Friends with the haircut.” People came up with lots of faces during the first minutes of the test, but the rate of remembrance dropped over the course of the hour. By graphing this relationship and extrapolating it to when most people would run out of faces, the researchers estimated the number of faces an average person can recall from memory.

To figure out how many additional faces people recognized but were unable to recall without prompting, researchers showed the participants photographs of 3441 celebrities, including Barack Obama and Tom Cruise. To qualify as “knowing” a face, the participants had to recognize two different photos of each person. By combining these two numbers and canceling out faces that appeared in both sets, the researchers determined the average person knows about 5000 faces, they report today in the Proceedings of the Royal Society B. All 25 participants in the study recognized between 1000 and 10,000 faces.

Although the experiment made a number of assumptions about the rate of remembrance—and relied on the honesty of participants—the results provide a baseline number for future facial recognition studies, the research team says. Next, they hope to explore why certain people (including so-called “superrecognizers”) can recall more faces than others. In the meantime, don’t be surprised if, when flipping through your mental Rolodex, there are a lot more faces in there than you realized.