How big are the differences between the hand-written copies of the Gospels' accounts of Jesus? Do these undermine what we can know about…

How many hand-written copies of the Gospel accounts are there? What does this reveal about possible mistakes?

Are the New Testament books an arbitrary or personal selection of what was available? Or is there more to it than that?

Were the Gospel accounts based on the testimony of eyewitnesses who were still alive when the Gospels were written?