Note the "For" in
Matthew 18:20. It is gar in Greek and is a preposition indicating causation, and can be and sometimes is translated "because". So, reading
Matthew 18:20 as "Because where two or three are gathered together in my name, there I am in the midst of them" makes one realize that
Matthew 18:20 is a conclusion on all the material that came before, and the material that came before is about church discipline, not a worship based communual meeting.
http://biblehub.com/interlinear/matthew/18-20.htm
http://biblehub.com/greek/1063.htm
So it's pretty clear
Matthew 18:20 doesn't have anything to do with the saints getting together to praise the Lord and edify each other. Consider the following parable, which is all about forgiveness and reconciliation, and what happens when a brother doesn't forgive a brother. It's a reinforcement of the principle regarding church discipline.
There isn't any one Scripture that says how many it takes to qualify a meeting of the church. It could be two or three, it could be something else. But the Bible is silent on the matter, and
Matthew 18:20 doesn't offer any additional insights, since it's not what the verse, or even passage, or even entire chapter, is about.