How many disciples followed Jesus at the peak of His ministry? Or more precisely, how many were there at the time of John 6:66? “After hearing this many of his disciples turned back and no longer walked with him”
That will depend on what you mean by “disciple” and by what you mean by following Jesus. From Matthew and Mark we get the impression that tens of thousands followed Jesus at the peak of his ministry. Five thousand men followed Jesus all the way around to the far side of the Sea of Galilee. If we count women and men, then there were more than ten thousand in this crowd. This is the context for the statement in John 6–the aftermath of the feeding of the 5000. We can assume that when Jesus ministered in Capernaum and in the Jewish heartland in Galilee, surely the crowds were even larger. Several tens of thousands heard Jesus and followed him around at the peak of his ministry.
But how many of these who were attracted to Jesus as a great preacher or as a worker of miraculous healings were actual disciples of Jesus? How many would have stayed with Jesus when he said in Luke 14:23-33 that only those who give up everything can be a disciple of Jesus? How many would have stayed when he challenged them, as found in John 6:66? We should assume that only a fairly small proportion of the tens of thousand who followed Jesus around had been converted and were true disciples of Jesus.
Again, in Jerusalem, many thousands listened to Jesus as he spoke, but how many were disciples? We do not know. What we do know is that after the crucifixion, when the true followers gathered in an upper room, there were only dozens, not thousands. However, the group in the upper room was probably not all of those who had committed to Jesus up to that time. According to Paul in 1 Corinthians 15:1-5, there were at least five hundred who saw Jesus alive after he was resurrected. Probably most of these were true disciples. The honest truth is that we do not know how many true disciples of Jesus were following Jesus either literally, or had committed to him during his ministry. Given the response on the Day of Pentecost in Acts 2, we can assume that the total number of real disciples of Jesus at his death was in the thousands, not in the dozens or even in the hundreds. But, we simply do not have this number available to us.
Sorry for a somewhat vague response.