Most standard 6-foot street benches are not insanely heavy, but they are not light either. In real-world terms, you are usually looking at roughly 150 to 300 pounds fully assembled. The exact number depends mostly on the material.
A typical steel-frame bench with wood slats tends to run about 180 to 250 pounds. That is the most common style you will see in parks and on sidewalks. All-metal benches are usually heavier, often 250 to 350 pounds depending on steel thickness and design. Recycled plastic benches can be all over the place, but many land in the 200 to 300 pound range. Concrete or granite benches are a different animal entirely and can easily exceed 400 pounds.
If your crew is moving a standard wood-slat and steel bench, two strong people can usually handle it, especially if you use lifting straps or a furniture dolly. For anything in the 250-plus pound range, I would plan on at least three to four people or use a hand truck with straps. Also remember to lift with your legs, keep the bench close to your body, and have a clear path before you start. When in doubt, remove the legs or slats if they are bolted on, because shaving off even 30 to 50 pounds makes a big difference on a long carry.