The short answer is 18 to 24 inches between individual outdoor benches when you line them up side by side in a row. That gap gives people enough elbow room, prevents armrests from hitting each other, and still keeps the row looking intentional rather than scattered.
If the gap between benches is also meant to be a walkway or accessible route, do not treat it as a small crack. Leave at least 36 inches of clear space, and 42 to 48 inches is even better if you expect two people to pass or a wheelchair to turn comfortably. This is the measurement I use for public parks, patios, and commercial spaces where ADA access matters.
For a tight visual row against a wall or storefront, you can go down to about 12 inches if the benches are armless and the gap is not a walking path, but I would not go tighter than that. It becomes a debris trap and a pinched-finger hazard. With benches that have armrests or contoured seats, stick closer to 24 inches so people do not feel like they are sitting on top of each other.
If you mean benches placed in a line along a walkway rather than touching side by side, the spacing changes. A common comfortable rhythm is to place them every 15 to 20 feet along the path, or closer in areas with older adults or people with limited mobility. That gives people a visible rest point without creating a cluttered run of furniture.
A good rule of thumb: for side-by-side spacing, use 18–24 inches between bench ends; for walkway gaps, use 36–48 inches; and for absolute minimum clearance in a non-public visual row, do not go below 12 inches. That balance keeps the area comfortable, safe, and easy to maintain.