Egyptian Creation Myth

I heard many Ancient Egyptian myths, but why were there so many? and Why were there so many Gods and Goddesses?

You got to bare in mind that Egypt at one time was not a unified state.  Just about each "nome" was a virtually independent state of its own with its own titular god or religion.  These statelets coalesced into two, one where Horus was supreme, and the other where Seth was supreme.  The myths say that Horus was the eventual winner, and as a matter of fact, we know the earliest Egyptian kings by their "Horus-names".   It is for this reason that we have conflicting Egyptian cosmologies and spiritual order of things.  At a certain period such-and-such god was the head god; at another period another god was the head god.  The Egyptians were well aware of these contradictions but they accepted them anyways.  Some of the gods even took on the characteristics of other gods.  It is therefore not really surprising if one finds more than one account as to how the Egyptians saw their universe formed.

Yes, Amun only comes into prominence when Thebes became the capital.  Before that, Ptah was preeminent because of the position of Memphis.

Doubtful.  Either they were Christianized or Islamized.