Why don't all planets have one moon like the earth? How do other planets aquire more than one moon?
The leading theory for how the Earth's moon was formed is the giant impact hypothesis, which states that the Moon formed after a large impact with another planet-sized object early in the Earth's history. The impact caused material to be blown off the surface of the Earth, and this material eventually coalesced to form the Moon. This theory may also apply to Pluto and its moon Charon.
Phobos and Deimos, the two moons of Mars, are thought to be captured satellites, which means that they were not formed where we see them today. They most likely originated in the asteroid belt of our solar system, which is located between the orbits of Mars and Jupiter.
The moon systems of Jupiter and Saturn are thought to have formed in a way similar to the formation of the solar system. The major satellites formed in a disk that orbited around the forming planet.
I hope this answers your question!
Image credit: Kenneth Seidelmann, U.S. Naval Observatory, and NASA