As Jesus is traveling through the countryside, he has compassion on the people "because they were harassed and helpless, like sheep without a shepherd" (Matthew 9:36). As a result, he tells his disciples, "The harvest is plentiful but the workers are few. Ask the Lord of the harvest, therefore, to send out workers into his harvest field" (9:37).
I wonder what people think of this. Do you see the plentiful harvest? I tend to see the problem and think about needing more sowers. Nevertheless, this is a specific item Christ asks us to pray for. Note though that as you ask for more workers you are probably signing yourself up either to be involved in equipping them or being sent out yourself. This is what happens to the 12 anyway.