You'd be surprised how quickly you go through them if you're building a large bridge. For example, I just realized I probably need almost twice as many as I initially ordered.
Remember, generally, each girder in a bridge has one whenever it rests on a structure, and girder bridges frequently have multiple reinforcing girders "inside":
https://goo.gl/maps/z54BVGaVFhyThen, you need one for each end of each girder. Piers where two girders meet typically have two on them, not one as I had initially modeled.