The mystery of the seven stars that you saw in my right hand and of the seven golden lampstands is this: The seven stars are the angels of the seven churches, and the seven lampstands are the seven churches.
It goes on for a bit and then all hell breaks loose. Come on, you know Revelation as well as I, certainly.
They were not different "churches" in the sense of a separate Church, but a separate parish or diocese. They were all part of the One Holy Catholic and Apostolic church, and they all answered to the same authority.
St. Paul also wrote to seven churches. Ephesus is the only one included in both lists . . .