What is the historical basis for the claim that there were hundreds of eyewitnesses to the resurrection of Jesus?
The main sources which directly attest the fact of Christ's Resurrection are the Four Gospels and the Epistles of St. Paul.
The fact of Christ's resurrection
The main sources which directly attest the fact of Christ's Resurrection are the Four Gospels and the Epistles of St. Paul. Easter morning is so rich in incident, and so crowded with interested persons, that its complete history presents a rather complicated tableau. It is not surprising, therefore, that the partial accounts contained in each of the Four Gospels appear at first sight hard to harmonize. But whatever exegetic view as to the visit to the sepulchre by the pious women and the appearance of the angels we may defend, we cannot deny the Evangelists' agreement as to the fact that the risen Christ appeared to one or more persons. According to St. Matthew, He appeared to the holy women, and again on a mountain in Galilee; according to St. Mark, He was seen by Mary Magdalen, by the two disciples at Emmaus, and the Eleven before his Ascension into heaven; according to St. Luke, He walked with the disciples to Emmaus, appeared to Peter and to the assembled disciples in Jerusalem; according to St. John, Jesus appeared to Mary Magdalen, to the ten Apostles on Easter Sunday, to the Eleven a week later, and to the seven disciples at the Sea of Tiberias. St. Paul (1 Corinthians 15:3-8) enumerates another series of apparitions of Jesus after His Resurrection; he was seen by Cephas, by the Eleven, by more than 500 brethren, many of whom were still alive at the time of the Apostle's writing, by James, by all the Apostles, and lastly by Paul himself.
Here is an outline of a possible harmony of the Evangelists' account concerning the principal events of Easter Sunday:
The holy women carrying the spices previously prepared start out for the sepulchre before dawn, and reach it after sunrise; they are anxious about the heavy stone, but know nothing of the official guard of the sepulchre (Matthew 28:1-3; Mark 16:1-3; Luke 24:1; John 20:1).
The angel frightened the guards by his brightness, put them to flight, rolled away the stone, and seated himself not upon (ep autou), but above (epano autou) the stone (Matthew 28:2-4).
Mary Magdalen, Mary the Mother of James, and Salome approach the sepulchre, and see the stone rolled back, whereupon Mary Magdalen immediately returns to inform the Apostles (Mark 16:4; Luke 24:2; John 20:1-2).
The other two holy women enter the sepulchre, find an angel seated in the vestibule, who shows them the empty sepulchre, announces the Resurrection, and commissions them to tell the disciples and Peter that they shall see Jesus in Galilee (Matthew 28:5-7; Mark 16:5-7).
A second group of holy women, consisting of Joanna and her companions, arrive at the sepulchre, where they have probably agreed to meet the first group, enter the empty interior, and are admonished by two angels that Jesus has risen according to His prediction (Luke 24:10).
Not long after, Peter and John, who were notified by Mary Magdalen, arrive at the sepulchre and find the linen cloth in such a position as to exclude the supposition that the body was stolen; for they lay simply flat on the ground, showing that the sacred body had vanished out of them without touching them. When John notices this he believes (John 20:3-10).
Mary Magdalen returns to the sepulchre, sees first two angels within, and then Jesus Himself (John 20:11-16; Mark 16:9).
The two groups of pious women, who probably met on their return to the city, are favored with the sight of Christ arisen, who commissions them to tell His brethren that they will see him in Galilee (Matthew 28:8-10; Mark 16:8).
The holy women relate their experiences to the Apostles, but find no belief (Mark 16:10-11; Luke 24:9-11).
Jesus appears to the disciples, at Emmaus, and they return to Jerusalem; the Apostles appear to waver between doubt and belief (Mark 16:12-13; Luke 24:13-35).
Christ appears to Peter, and therefore Peter and John firmly believe in the Resurrection (Luke 24:34; John 20:8).
After the return of the disciples from Emmaus, Jesus appears to all the Apostles excepting Thomas (Mark 16:14; Luke 24:36-43; John 20:19-25).
The harmony of the other apparitions of Christ after His Resurrection presents no special difficulties.
It is not impossible, that Christ appeared to other after the Resurrection. Catholic tradition first appeared to his Mother Mary.
“The Gospels mention various appearances of the risen Christ, but not a meeting between Jesus and his Mother. This silence must not lead to the conclusion that after the Resurrection Christ did not appear to Mary; rather it invites us to seek the reasons why the Evangelists made such a choice,” - St. John Paul II, the great Marian saint, told everyone at a general audience May 21, 1997.
Even the Evangelist, St. John admits the the Gospels do not contain everything Christ did and taught, so one can speculate that Christ appeared to others and that there may have been more than just two Angels that were at the tomb when the women went there. Only two were physically seen during that event! Other Holy Angels could have been present.
25 There is much else besides that Jesus did; if all of it were put in writing, I do not think the world itself would contain the books which would have to be written. - John 21:25