If you follow NY Gov Cuomo conferences they have addressed this, I've been following NY & NJ Gov's news conferences because I figure what they are going through now is what we here in midwest will be experiencing soon.
Short version all the models they have from CDC etc for peak wave of people needing medical intervention suggest they will need that many vents.
They are keeping the bulk of them in centralized storage because it functions akin to military reserve forces that you keep back to plug into the breaches.
There is no way to know ahead of time if this hospital will need 100 vents and that one 150.
HTH