I would like to know how a Christian should react to government requests to fil out census forms.
"Let every soul be subject to the governing authorities. For there is no authority except from God, and the authorities that exist are appointed by God. Therefore whoever resists the authority resists the ordinance of God, and those who resist will bring judgment on themselves" (Romans 13:1-2).
There is nothing wrong or immoral about filling out a census form. There were census in the Old Testament, which is were the book of Numbers came from, and even Jesus' own parents participated in a census. "And it came to pass in those days that a decree went out from Caesar Augustus that all the world should be registered. This census first took place while Quirinius was governing Syria. So all went to be registered, everyone to his own city. Joseph also went up from Galilee, out of the city of Nazareth, into Judea, to the city of David, which is called Bethlehem, because he was of the house and lineage of David, to be registered with Mary, his betrothed wife, who was with child" (Luke 2:1-5).
It is true that God did not want David to number his people. We aren't told exactly why, but there really was no reason for David to take such a census unless he was planning something that didn't have God's approval. But Israel did have a census tax, so there was an indirect way of number the people available to the king (Exodus 30:12).