This likely does not refer to all of Egypt but specifically to the land of Goshen where Israel had settled (Genesis 47:4, Genesis 47:6, Genesis 47:27; Genesis 50:8). The strongest evidence for this comes from Exodus 8:22 and Exodus 9:26—both indicate that Israel remained within the land of Goshen at time of the plagues.
7 But the people of Israel were fruitful and increased greatly; they multiplied and grew exceedingly strong, so that the land was filled with them.