Why do we have only four gospels (Matthew, Mark, Luke, John) if there were twleve apostles? If there were more (i.e. Gospel of Thomas), what happened to the others?

If you care about the number, well, then you must watch the excellent movie ZEITGEIST at www.zeitgeistthemovie.com it's free.
The new testament is plagued with astrological references.
Number four is a recurrent sacred number in Babilonian astrology and religions with all sort of powerful meanings. Four are the cardinal points, and four are the beasts in visions of Ezekiel, four the beasts and riders of revelation (Apocalypse) and so forth. The four beast of Ezekiel are taken from the four guardians of the Assyrian temples (normally shown as a composite beast): a human (for Mathew), a lion (for Mark), the ox (for Luke), and the eagle (for John).
If the Jews (Ezekiel among them) were captive or slaves of the Assirians, the origin and references are quite clear.
Is it only a coincidence that the Romans chose four and then gave the so called evangelists the same symbols of Ezekiel?

Actually, there were many accounts of the life of Jesus made by apostles, friends and observers present around that time. However, only Matthew, Mark, Luke and John's accounts were made official canon of the Catholic Church when the bible was assembled centuries ago. It should be noted that as time passed and sects broke from the Catholic Church, Protestant Christian sects often disagreed about which books and translations would be considered Canon for their Christian following. The rejected accounts are widely considered as "New Testament Apocrypha" and include such writings as the "Gnostic Gospels."

Many other gospel accounts exist, some attributed to significant people in the life of Jesus, such as Thomas, St. James, etc, though we cannot say with certainty today that the attributed authors had anything to do with the account. These gospels were not included in the Catholic canon for various reasons. Some of these Gospels present accounts of different aspects of Jesus' life and some suggest major differences with primary Church teachings. I will not go into these because it is important to note that these secular accounts were evaluated by scholars of the Church in earlier times and deemed to be inaccurate accounts of Jesus' life. I wish not to spark a religious argument with this post so I will leave my comments at that.

Other Gospels are published under different editors and translations and are readily found at any major bookstore. All I can suggest is that if you do pick one up, read it with a grain of salt with the understanding that there were reasons why these gospels were not included as canon in the bible of most Christian sects.

I look at it this way, God made sure we had what we needed....HE knows tendancies of men...it will ENDURE until the END comes...

First of all, the authorship of some of the Gospels is uncertain. Mark and Luke were never Apostles, though Mark was an associate of Peter's and may have written down his friend's stories. Luke retold the stories for a Greek patron; I don't know anything about him personally, though. Matthew's Gospel may or may not have been written by the Apostle Matthew, although it's generally thought that it wasn't. It may have been based on an earlier Apostolic account, though--the hypothetical Q document upon which Mt, Mk and Lk were based.

John was probably actually written by the apostle John, or someone wanted the world to think that it was (not to be too cynical), just because of the way he referred to himself as "that apostle."

There are other Gospels attributed to other apostles, but few of them have the same kind of literary standard as the canonical works. Thomas is simply a collection of sayings, much like the Analects of Confucius or the Book of Proverbs. I don't know if it's considered to have been written by the Apostle Thomas to whom it is attributed. Judas is pseudopigraphia written by much later gnostics and is definitely not by any of the Judas' who followed Jesus (Thaddeus and Thomas were also named Judas).

Maybe they just didn't "cut the mustard" or due to limited space they just kept the better ones. What I don't get is why the four they kept are so similar.

four winds (cardinal points on the compass).... four gospels. (no more no less)