Does that mean half of those managers will become engineers?

If I were a betting person, I'd bet that's the number of engineers who were previously forced to become managers in order to get a promotion.

I've worked with a number of people who made the IC -> manager conversion because it was represented as the best way forward in their career, only to find out it made them miserable, and convert back after a few years. I think you'll find that sort of conversion back to IC is not all that uncommon.