How many Terrestrial planets have Iron Nickel cores?
All four terrestrial planets Mercury, Venus, Earth and Mars have iron-nickel cores. Mars may have some iron sulfide mixed in.
Iron and nickel are the most common elements in the metallic matter of our Solar System (as opposed to the lighter elements predominating in rock and ice). So they are the major elements of the dense metallic matter that sank to the cores of the terrestrial planets when they differentiated.
Our moon and Jupiter's moon Io have iron-rich cores as well, for the same reason. Some asteroids have predominantly iron-nickel compositions inherited from the cores of differentiated bodies that later were broken up.
However, as we go farther out in the Solar Syatem we tend to see less metal, more rock and ice; that is, the dense metallic material is predominantly close to the Sun. Apparently the original solar nebula was partially differentiated.