Answer:
There are no recorded, definite Ambien)zolpidem) fatalities at this time. The LD50 for rats is astronomical, and so much so that extrapolation to humans is questionable. Some anecdotal evidence talks of cases admitted with 4g zolpidem dosages that slept for a few days then recovered. So, in direct answer, the number is not known but will be extremely high.
This however doesn't mean that megadose of Ambien are safe. There are quite a few surprising side effects, including but not limited to amnesia, sleep walking, sleep eating, promiscuity (!), dizziness and others.
Ambien is a surprisingly safe drug and not easily lethal, but still merits caution in administration.