Magi-why does everyone always assume their is three of them?

Im not really sure why everyone assumes their where three magi, lets look at Mathew 2: 1-8 Jesus was born in Bethlehem in Judea, during the reign of King Herod. About that time some wise men from eastern lands arrived in Jerusalem, asking, ² “Where is the newborn king of the Jews? We saw his star as it rose, and we have come to worship him.”  King Herod was deeply disturbed when he heard this, as was everyone in Jerusalem. ⁴ He called a meeting of the leading priests and teachers of religious law and asked, “Where is the Messiah supposed to be born?”  “In Bethlehem in Judea,” they said, “for this is what the prophet wrote: ‘And you, O Bethlehem in the land of Judah,are not least among the ruling cities of Judah, for a ruler will come from you who will be the shepherd for my people Israel. Then Herod called for a private meeting with the wise men, and he learned from them the time when the star first appeared. ⁸ Then he told them, “Go to Bethlehem and search carefully for the child. And when you find him, come back and tell me so that I can go and worship him, too!

We always see the seen the baby Christ is in the manger and three magi (well their is some rebellion to that now, but most people still want to see even if they don't follow our Lord). Also what is meant by the nativity seen, it the birth seen and comes from Latin. We see Jesus's conception in Luke 1: 26 it was announced 6 months into Elizabeth's conception with John the Baptist. Jesus was born slightly before the death of Harrod (the King at the time).

"The Magi also referred to as the three Wiseman where a group of distinguished foreigners who visited Jesus after his birth, bearing gifts of gold, frankincense and myrrh." (From Wikipedia).  I suspect they had some sort of government role as we get the modern word magistrate. Three Wiseman, they came with a large army. They may have been three at the top of chain, or their could be a lot more than three. The followed a star to the newborn baby Jesus due to the prophecy in Isaiah. They arrive shortly after the birth of the savior of the world and give the baby Jesus  gold, frankincense and myrrh. Despite the criticism of the day Christmas is on, it is believed that they discovered him on December 25th.