Two

Two is the number of witness. For major events, two witnesses were required:

"At the mouth of two witnesses, or three witnesses, shall he that is worthy of death be put to death; [but] at the mouth of one witness he shall not be put to death." Deuteronomy 17:6

Jesus, describing the process for church discipline, said:

"But if he will not hear [thee, then] take with thee one or two more, that in the mouth of two or three witnesses every word may be established."

This principle of witness also plays a role in the trinity, which I will discuss more at 3.

Matthew 3:16-17:
"And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him: And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased."

At Jesus' baptism, the Holy Spirit and the Father bore witness to the veracity of His message.