The eyewitness pool

1 Inasmuch as many have undertaken to compile a narrative of the things that have been accomplished among us, 2 just as those who from the beginning were eyewitnesses and ministers of the word have delivered them to us, 3 it seemed good to me also, having followed all things closely for some time past, to write an orderly account for you, most excellent Theophilus, 4 that you may have certainty concerning the things you have been taught (Lk 1:1-4).

6 Then he appeared to more than five hundred brothers at one time, most of whom are still alive, though some have fallen asleep (1 Cor 15:6).

What was the total eyewitness pool? Here's an estimate:

Assuming the Gospel of Mark (our earliest Gospel) was composed around thirty years or so after Jesus’ life and ministry (ca. AD 65), how many eyewitnesses would have been alive to consult during the research and writing process? And beyond this, how many would have been alive when the last Gospel (John=AD 90?) was written? McIver brilliantly looked at the latest research in population size around Galilee, Jerusalem, and the other villages and cities Jesus visited during his ministry in antiquity, and what life expectancy was in the first century in Roman-Palestine. He concluded there would have been approximately 60,000 potential eyewitnesses who saw or experienced Jesus in person. McIver claims that “[o]f the 60,000 or so potential eyewitnesses, between 18,000 and 20,000 would be still alive after thirty years, and between 600 and 1,100 after sixty years.” (4) He concludes the book by stating that “…as is evident from the life tables, some surviving eyewitnesses would have been available to the Evangelists to consult had they so wished.” (5) This is very important information for anyone interested in the possibility that the Gospels were either composed by eyewitnesses or depended on the tradition of eyewitnesses to Jesus’ life and ministry. Assuming the standard dating for the composition of the Gospels (Mark=AD 65, John=AD 90) it would appear there were in fact many eyewitnesses of Jesus' ministry to consult if the Gospel writers desired. 

We actually have a quotation from the work of an early Christian apologist named Quadratus (ca. 70-130 AD) (6) who claimed that eyewitnesses of Jesus' ministry (people who were healed by Jesus) actually lived well into the later part of the first century:

"But the works of our Savior were always present, for they were true; those who were healed and those who rose from the dead were seen not only when they were healed and when they were raised, but were constantly present, and not only while the Savior was living, but even after he had gone they were alive for a long time, so that some of them survived to our own time." Greg Monette, "Question: Were Eyewitnesses Alive for Matthew, Mark, Luke, and John to Consult?"