By the time John wrote Revelation there were 7 churches. And from the Book, John penned what their ailments were and still are.
Those were local churches belonging to the one Church that Christ establshed.
Yes, there were at least 7 congregations of believers, 7 ecclesiae if you will. There were many more congregations however. For instance, the believers in Jerusalem, Philippi, and Thessalonika were not mentioned in John’s writing, amongst others.
Perhaps the question would be better asked - from whence does any church derive its authority? If from someplace other than the Word, why?