Probably too early to tell. Early reports have it at like a million which would be a drop in the bucket. I don’t think Twitter would die just because Elon took over, the question is will his changes overall slowly improve the site or slowly degrade it for “most people” whatever that means, the hard part for Elon is the least sticky twitter users may also be the least vocal. He could listen to everyone and still lose a large swath of users for reasons he doesn’t understand.
The loudest people may be leaving Twitter for now, but I doubt it will be the death of the platform.
But depending how things change it might happen.
Sure, some will leave, but alternate platforms will never come close to even Twitter's tiny numbers.
I've heard that a ton of people have actually joined over the last week. Net new users apparently.