They can also be used on the chain stay bridge if it lacks a fender-bolt hole.

The price is for one clamp. Clamps are sized based on the diameter of the tube they are going around. If you don't have calipers to determine this, take some string and wrap it around the tube. Measure the distance and divide by 3.14 to get the diameter.

Metric equivalent sizes: 1-1/4" = 31.5mm, 1"= 25.4mm, 7/8" = 22.2mm, 3/4" = 19.1mm, 5/8" = 15.9mm, 1/2"= 12.7mm, 3/8"= 9.5mm, 1/4"= 6.4mm