The short answer is that a standard outdoor bench can comfortably seat two to three adults, depending on its length and design. Most standard park or garden benches are between 4 and 6 feet long, which is roughly 48 to 72 inches. A 4-foot bench is usually best for two adults, while a 5-foot to 6-foot bench can typically handle three average-sized adults without anyone feeling too squeezed.
In terms of personal space, a good rule of thumb is to allow about 24 inches of seating width per adult. That means a 48-inch bench gives two people a comfortable amount of room, a 60-inch bench can seat two people very comfortably or three people a bit more snugly, and a 72-inch bench can generally seat three adults without much overlap. Armrests, backrest angles, and cushioning can also reduce the usable seat space, especially if the armrests divide the bench into defined spots.
Kids can fit more easily because they take up less width, but for adults shoulder width and personal boundaries play a big role. If you want guests to feel genuinely relaxed, plan for at least two feet per person. So while you might technically squeeze three people onto a small bench, the honest answer is that two to three adults is the realistic comfort range for a standard outdoor bench, with three being much more comfortable on the longer end of that size range.