This set gives you 2 lengths of 10cm. You will generally require two sets, 2x2 ie 4 legs, but if you are getting two Shelfs side-by-side you will require three sets, 3x2 ie 6 legs.
The main use of this size leg is when you are having a slatted shelf for pots and you want it relatively close to the ground but still enough room to clean under.