What does your scripture say? Where from so many billion souls are coming ? I think God has a Birth Ministry where souls are asked to choose an egg, when the egg is chosen, the soul is thrust into the egg
Assuming we all exist at all times regardless of our physical body we know now, it is possible what we consider to be souls could have existed as some other form of energy. Do you believe we are simply recycling the same souls over and over again in birth and why would something already alive be sitting around waiting to be born? Somehow, I think the idea of reincarnation to a different human being to be so intrusive, I don't know. I think you would be better off existing as pure energy because you would not be restricted by a physical body.
I don't think with Christianity anyone knows the exact moment when a person's soul is created. Some time during the procreation process at least. But the sould is created by God at that time he chose, and a soul is unique to each person and it is forever. In Genisis it talks about God breathing life into Adam, and I like to imagine that our soul is a little part of God's being that is attached to our body until we die. If that's correct, then our soul was part of God ( who is infinite) before we were born, but once in the flesh it is currupted by original sin and won't be made perfect again until we are back with Him in heaven. I don't think there are any real clear specifics on this in Christianity though.