Rock grips come in a set of 12. Plastic blow molded Rocks are molded from polyethylene with UV and cold weather protection. There are two different sizes of mounting holes. The attaching screws and bolts are not included. Now smaller sized with the bolt holes are 3/8" and 2" center to center.
Install using two different size screws. Screws are not included with set of rocks. You will need to get 12 wood screws 3/8" x 1 ½" lag bolts and 12 wood screws #14 x 1 ½".