Perhaps it will help to consider Matthew 18:20 in its context:
Matthew 18:15-20 (New King James Version)
15. “Moreover if your brother sins against you, go and tell him his fault between you and him alone. If he hears you, you have gained your brother. 16. But if he will not hear, take with you one or two more, that ‘by the mouth of two or three witnesses every word may be established.’ 17. And if he refuses to hear them, tell it to the church. But if he refuses even to hear the church, let him be to you like a heathen and a tax collector.
18. “Assuredly, I say to you, whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven.
19. “Again I say to you that if two of you agree on earth concerning anything that they ask, it will be done for them by My Father in heaven. 20. For where two or three are gathered together in (Grk. eis) My name, I am there in the midst of them.”
In v. 17 we find the "church" twice mentioned. It seems obvious the meaning is the local assembly. The "two or three" in verse twenty informs us that even that small number may be considered "church". However, two or three people, or even hundreds of people gathered together, do not constitute "church". People gathered together to play games or watch movies and eat popcorn are not "church". Intentionality seems to make the gathering "church".
When Matthew wrote these words, he did not use the Greek preposition en which is often used with Jesus' name [ask in (Grk. en)my name, etc.], but he used the preposition eis. Eis denotes motion to or unto an object, with the purpose of reaching or touching it. From this comes the idea of the object toward which such motion is directed and for, or with respect to which such action or movement is made.
So I am thinking the number gathered, and the location, are irrelevant to constituting "church". What matters is that we gather intentionally with respect to and for Jesus through whom we worship the Father. After all, its His assembly (church).

Just my thoughts.
God bless, Homer