It is very hard to reach human lethal dosage with Ambien. Mice were dosed to 80mg/kg/day in mutagenics study and did just fine. This is the equilvalent of well over 10 times the recommended daily adult dose. And while extreme overdoses have produced coma, I can't find a single death.
In short, I can't determine what the lethal dose is for ambien. There is an insufficient number of fatalities to make this assessment. However, the dose is a lot higher than you may think.
You're advised to stick to the prescription with this medication.