A lot.
the answer is 45
the rectangular things on the side are called trusses. so there for the "truss" bridge
A truss is a diagonal brace which provides structural support for a bridge span by extending between a vertical member and the bridge span.
yup
As many as it takes. Truss bridges should have triangles all over. Openings that are not a triangle (that have 4 or more sides) are not stable.
Visit the Glienicke Bridge in Berlin, Germany
Golden Gate Bridge in San Francisco, California
Visit the Oresund Bridge in Denmark
Visit the Skinny Bridge in Amsterdam, the Netherlands