There was definitely a massive migration, at least seen from lemmy's side, check out the graphs at Fediverse observer. Within a two month span (May-July 2023), the number of total monthly lemmy users went from 47 000 to 1 700 000. That's a 37-fold increase in two months. The number peaked at about 1.9 million in august the same year, with about 130 000 active monthly users (up from about 1 000). In the same period, the number of online lemmy servers went from 80 to a peak at 1396 (a 17-fold increase).

In reddit terms, the migration wasn't massive, they lost on the order of a hundred thousand users. Still, over a hundred thousand users leaving a platform within a couple months is probably near-unprecedented in internet history.