Seven, or a multiple of seven, is found in the first two chapters of Exodus,
“All those who were descendants of Jacob were seventy persons (for Joseph was in Egypt already)” (Exodus 1.5).
“Now the priest of Midian had seven daughters. And they came and drew water, and they filled the troughs to water their father’s flock” (Exodus 2.16).
As with some of the other references we have seen, these do not appear to present any significance, other than simply what they state.
However, once we arrive at Exodus 7, we read of the first plague when the Lord turned the waters of Egypt into blood, and then Moses wrote,
“And seven days passed after the LORD had struck the river” (Exodus 7.25).
The Lord let them experience that plague for a week before moving on to the next one, and we need to see this because we might have the tendency to think that one plague just followed another, day after day, but right away we learn that time transpires between the plagues.
Even if the waters of Egypt were blood for only one day, the Lord still let them think about that event, and when a week had gone by, a little anniversary so to speak, He brought on the next one.