Just stopping by to clarify a common misconception about Presbyterian polity:
Presbyteries aren't groups of congregations; they are assemblies of elders, or presbyters (1 Timothy 4:14). To write to seven congregations, and to write to a presbytery that oversees those congregations, is not precisely the same thing.
Nevertheless, the Ephesian church alone was almost certainly made up of multiple congregations. As the Westminster Form of Presbyterial Church Government says:
That there were more congregations than one in the church of Ephesus, appears by Acts xx. 31, where is mention of Paul’s continuance at Ephesus in preaching for the space of three years; and Acts xix. 18, 19, 20, where the special effect of the word is mentioned; and ver. 10. and 17. of the same chapter, where is a distinction of Jews and Greeks; and 1 Cor. xvi. 8, 9, where is a reason of Paul’s stay at Ephesus until Pentecost; and ver. 19, where is mention of a particular church in the house of Aquila and Priscilla, then at Ephesus, as appears, Acts xviii. 19, 24, 26. All which laid together, doth prove that the multitude of believers did make more congregations than one in the church of Ephesus.