There is really no established lethal dose of clonazepam alone. However, mix the benzo with opiates or alcohol, as was previously mentioned, and it's a whole different ball game. If you are unfamiliar with your limits with either/or, or are just plain reckless, it can be lights out -- for good.
I had a very close call with a combo of methadone and diazepam about 13 years ago, and an even closer call with alcohol and diazepam about 6 years ago. The latter experience landed me in ICU with a ventilator keeping me alive for several days. I was noticeably out of it for a few weeks after waking up. Whether or not I have permanent damage is hard to say.
Moral of the story: benzos alone, nearly impossible to OD on, but can cause terrifying blackouts. Benzos mixed with opiates or alcohol, a great way to meet your maker if you're not careful.