In Sanhedrin 111a, we read:
It is taught in a baraita with regard to the few that are destined to be redeemed: Rav Simai says that it is stated: “And I will take you to Me as a people” (Exodus 6:7), and juxtaposed to that verse it is stated: “And I will bring you into the land” (Exodus 6:8). The Torah compares their exodus from Egypt to their entry into the land; just as during their entry into the land only two of six hundred thousand entered the land, as they all died in the wilderness except for Caleb and Joshua, so too, during their exodus from Egypt, in terms of the ratio, only two of six hundred thousand left Egypt and the rest died there. Rava says: And likewise, that will be situation in the messianic era, as it is stated: “And she shall respond there, as in the days of her youth, and as in the day when she came up out of the land of Egypt” (Hosea 2:17). The ultimate redemption and the exodus from Egypt are juxtaposed, indicating that in the messianic era too, only few will survive.
So, only two out of the 600,000 adult Jewish men entered the promised land (Joshua and Caleb). That's believable, after 40 years in the desert. But then we read that only 2 in 600,000 Jews left Egypt in the first place. This puts the number of Jews in Egypt at 600,000 x 300,000 = 180 billion. And that's just the adult males.
Now, I do not believe Rav Simai (supported by Rava) believed there were that many Jews. But I do believe that every statement in the Talmud is meant to teach us something. So what is the teaching here? (Other than the depressing thought that only 1 in 300,000 of us can expect to be redeemed in the future.)