Honestly, the short answer is two to three adults. A standard park bench is usually around 4 to 6 feet long, with most commercial models sitting right around 5 feet. That gives enough room for two people to sit with real personal space—elbows not touching, maybe a coffee or a bag between you. Three adults can fit, but it becomes friendlier than spacious. If everyone is average-sized and happy to sit shoulder to shoulder, three works fine. Four? Only if they are kids, or you all really like each other.
The seat depth and armrests matter too. Many standard benches have a seat depth around 18 to 20 inches, which is comfortable for most adults. If the bench has a center armrest, it usually turns a three-person bench into a two-person bench by design. Backless benches can feel more flexible because people can sit from either side, but the actual comfortable capacity stays about the same.
If you are choosing a bench for a park, school, garden, or commercial space, I would recommend planning for two adults per standard 5-foot bench if you want true comfort. If you need to seat three regularly, look at 6-foot models or benches without dividers. And for kids, you can generally fit three to four on the same bench because they take up less shoulder room.
So my rule of thumb: a standard park bench comfortably seats 2 adults, can seat 3 in a pinch, and is perfect for 3 to 4 children. When in doubt, size up—nobody ever complains about having a little extra room on a park bench.