John Gill Commentary Luke 10:2

"And he said unto them, The harvest indeed is plenteous, but the laborers are few: pray ye therefore the Lord of the harvest, that he send forth laborers into his harvest." — Luke 10:2 (ASV)

Therefore he said to them
That is, the "Lord Jesus", as the Ethiopic version expresses it; he said to the seventy disciples, what he had before said to the twelve apostles in (Matthew 9:37Matthew 9:38) where are the same words as here:

the harvest truly is great, but the labourers are few, pray you
therefore the Lord of the harvest that he would send forth
labourers into his harvest ;
for though there was such a number of disciples called to the ministerial work, and sent out, there was still need of more; so great was the harvest of souls, or number of hearers, that the labourers were yet but few; and therefore the Lord of the harvest and whose all souls are, was to be prayed to to send forth more laborious preachers; (See Gill on Matthew 9:37). (See Gill on Matthew 9:38).