In Acts 20, Luke mentions several guys who were accompanying Paul. All but one are obscure. Who has heard of Sopator, Aristarchus or Secundus? Who has heard of Gaius, Tychicus & Trophimus? We only have heard of Timothy. But they all came from Paul's ministry each from a different city.
Timothy and Gaius came from cities (Lystra and Derbe) where Paul launched churches on his first missionary journey with Barnabas. Sopator, Aristarchus and Secundus all came from two Macedonian cities (Berea and Thessalonica) where Paul launched churches on the 2nd Journey. Tychicus & Trophimus are from Asia. Asia is the district where Ephesus is the main city. Paul stopped by Ephesus at the end of his 2nd Journey and came back and preached there for while on his Third Journey. The disciples in Ephesus proclaimed the gospel throughout the Roman district of Asia.
What's the point?
Paul developed leaders (made disciples) but its not like he stayed in these cities very long. Very short times in Lystra where he was stoned and Derbe, three weeks in Thessalonica. The longest was Ephesus but even that is short. All of his journeys put together were done in under 10 years.
My friend Shane says it better than me: "I am probably way off my rocker, but it seems to me that you don't get leaders by focusing on leaders, you get leaders by focusing on the mission. Leaders are what bubble to the surface when the vision and mission are compelling enough to engage in risky behavior - thus needing leadership. As soon as we try to organize, program and teach our way toward leadership we are in trouble. Leaders are developed in the crucible of the mission focused adventure."
Paul's leadership seminary class was field work of launching movements through the Eastern part of the Roman Empire and then entrusting these young leaders to lead and develop others in the mission outposts to take the message elsewhere.