It's anecdotal, but a friend of mine working on the frontend told me his org of 150 engineers lost 2/3 in the original layoff (leaving 50), and then about 80% of those remaining declined the hardcore ultimatum (leaving 10, a mere 6.7% of the starting strength). H1B's are the majority of those remaining.
It was first 50% of the 7500 initial and then another 50% of the remaining. Then add all this Code Review nonsense and you can make the argument that at least 80% of the original are gone. I wonder how many he has brought in though. There is too much muddying of the waters to get a realistic estimate of people joining Twitter because they want to be part of Musk's "vision". It could be anything from hundreds of people joining to just 1 (George Hotz lol).