There is a beautiful and meaningful parallel between the rituals of the old covenant and of the new covenant. This is true of baptism in the new covenant.
Baptism is done once, when the person enters the covenant.
The old covenant also had sacrifices that were performed twice daily, weekly, monthly, annually and whenever the need arose. In the new covenant, the Lord's Supper replaces all those animal sacrifices, because Jesus' death fulfilled them all. We have freedom in Christ to observe the Lord's Supper as often as we wish to remember his death in this way. When we take the elements of his Supper, we are doing it in remembrance of him and proclaiming the significance of his sacrifice for us. Since Jesus began the Lord's Supper after having told his disciples to prepare for the Passover, it would seem reasonable to conclude that he used unleavened bread.