On why the Christ incarnated only once
From my own understanding of Steiner's Christology, the importance of the Jesus child of the Gospel of St. Luke (also referred to as the Nathan Jesus, because Nathan is the ancestor where Luke's genealogy diverges from Matthew) is the special quality of the ether body that he brought. Because the Ego of that Jesus was incarnating for the first time, it had no karma, and therefore no consequences of sin. Being, as it was, without the consequences of sin, it was undamaged and possessed none of the weaknesses that sin causes. Only thus could it even hope to contain the immense power that a God would bring to a human incarnation, and allow a divine ego inhabiting it to work in the world without hindrances and distortions. Were Christ to attempt to inhabit a human being with a less perfect ether body (say, mine, or for that matter any other random person you might imagine), one consequence would be that the intentions of the divine would not be able to be properly expressed, because the instrument, in this consideration specifically the etheric body, would be forever getting in the way. I should note that in such a hypothetical case, the astral and physical bodies would of course also be hindrances. To really understand these things, you have to go into the details from every angle.
For references to the above, see Rudolf Steiner's lecture cycle on The Gospel of St. Luke, especially the 7th lecture (September 21st 1909), and From Jesus to Christ, especially the 8th lecture (October 12th, 1911) As to there being no more souls, I think this concept too is best understood out of a deeper understanding of the being of man as presented by Rudolf Steiner. In An Outline of Occult Science (or Esoteric Science as it is being translated today) Steiner describes the processes that have created the human being as she/he is today. This is an incredibly long process, beginning as it does, at the near edge of eternity, and is already in the 4th major phase of development. If the process, as described there, makes sense to you, it should also become evident that more ego's could not simply pop into existence at divine whim. Now if we are dealing with a finite number of egos, who were all present in the beginning with God, then the question becomes how they all subsequently developed. Steiner describes the process in great detail, summarized in the book and Outline of Esoteric Science, but filled out to a complete picture of extraordinary richness in his lecture cycles, with virtually every lecture between 1904 and 1911 adding at least some new details. To shorten it all to one sentence, once the earth became suitable, the egos started incarnating on it, some sooner, some later, so that by the start of recorded history almost all had been incarnated at least once. At that point, the few that were still coming down for the first time bore extraordinary tasks, and the very last to come down was the Jesus of the Gospel of St. Luke.