This could be translated as "the number of days that God required."
## for their purification ##
"for them to become ceremonially clean" or "for God to consider them to be clean again"
## to present him to the Lord ##
"to bring him to the Lord" or "to bring him into the Lord's presence." This was a ceremony acknowledging God's claim on the firstborn children who were male.
This is an idiom for "firstborn offspring that is a male." The law was about the firstborn offspring of both people and domestic animals, but in this context it could be translated as "firstborn son." (See: [[rc://en/ta/man/translate/figs-idiom]])