Gimatria Im Hakollel

This week’s Parasha begins by saying. "It happened when Pharoh sent out the people" The Ba’al HaTurim comments that the Hebrew phrase "the people" is the Gematria of the Hebrew phrase "also a mixed multitude" [went up with them from Egypt] A footnote in the Davis Edition of The Ba’al HaTurim on the Chumash says the principle of Im Hakollel allows 516 to be considered equivalent to 517, the Gematria of the Baal HaTurim’s term. My understanding is that the concept of Im Hakollel is based on the unity of Hashem. Could you please explain this in more detail.

See Tur Orach Chayyim 117. The principle is based on the idea that the whole is more than the sum of its parts. The people-inclusive- that Pharoah expelled included the Erev Rav.