The expression the harvest is plentiful, but the labourers are few
does not refer back to the shepherding image of Matthew 9:36, but forward to the work of the disciples described in Matthew 10:1. He is calling on the disciples to assist him in his earthly ministry with, like his work, healing. Jesus is also here not emphasizing the final harvest itself (Revelation 14:15), but the angelic (Matthew 13:30–39; Mark 4:29), and human (John 4:35) labour involved in reaping the harvest. The Old Testament already prophesied that the last days will be a time of harvest, bringing both salvation in the gathering of the children of God (Isaiah 27:12) and judgment (Joel 3:13), as will be seen in Matthew 10:1–42.
The image of the harvest is also typically used in reference to the final judgment. And in the context here in Matthew 9:37, the evangelist is speaking in the first place about distributing the powers of healing to the people (Matthew 9:35, Matthew 10:1). If Jesus intended to speak about the flock in Matthew 9:37, switching to this image would make little sense. Why would he not continue with the image of the large flock that needs shepherds to gather them in (as he did in John 10:1–2). When Jesus here laments that the work pertaining to the harvest is too much for himself, he is strictly referring to the limitations of his personal human nature which prevent him from going around to all the villages, healing people all day and night. However, the source of his power never dries up.1
The saying must not be understood as contrasting the great harvest with the few labourers (which at the time would have actually just been Jesus himself). Jesus is here, in Matthew 9:37, not speaking about the yield of the harvest (as in Revelation 14:15), but rather about the labour required in the act of harvesting itself (as in Matthew 13:30–39;Mark 4:29; John 4:35).
In the Old Testament it was already prophesied that the last days will be a time of harvest, which would entail both salvation in the gathering of the children of God (Isaiah 27:12), as well as judgment (Joel 3:13). That this is the time that Jesus is referring to here will also become evident in Matthew 10:1–42.2
37 Then he said to his disciples, “The harvest is plentiful, but the laborers are few;