How many? in my point of view, none will be gone by next year. I sincerely agree with Michael just about every time, but not this time.
PhaseOne is the technological leader of this niche (I hate this term). And now closely working with Mamiya, they will only get more atractive for a wide gamma of photogs in every level of expertise and budget power. They are not going.
Hasselblad is the commercial leader in digital medium format. Totally surrounded by immense marketing and the best support. They are not going either.
Leaf, Sinar, Rollei and F&H just invested in a new system that defies Hasselblad's own guts. If I had to think about who's at stake, it would be LSRF&H. Their new system is amazing but horribly expensive. but in a world where a company like Yahoo is worth the GNP of 20 countries, 10K usd entrance fee (film) can be money change for some and a few more. So, I think they will succeed and will get profitable in short.
Who else? Digital medium format Fuji is in crio, dmf Pentax is sleeping a long winter in Transilvania.
Those gone will be missed. Those alive today will be here for the next 50 years, at least.