That Fifty Men were selected to carry the Cross
Accordingly, he selected those of his bodyguard who were most distinguished for personal strength, valor, and piety, and entrusted them with the sole care and defense of the standard. There were thus no less than fifty men whose only duty was to surround and vigilantly defend the standard, which they carried each in turn on their shoulders. These circumstances were related to the writer of this narrative by the emperor himself in his leisure moments, long after the occurrence of the events: and he added another incident well worthy of being recorded.
Source: Life of Constantine (New Advent)