People die by suicide with benzos all the time, but there's not much data out there (at least publicly) on how much it takes to kill someone. Even then, it probably varies from person to person based on things like age, weight, metabolism, preexisting physical issues, etc. Benzo overdoses are unreliable too, so they haven't been looked into and investigated as much, for example, SN. We don't have any charts or guides for ctbing with benzodiazepines. The info just isn't out there.
So we also don't know a lot about how painful the process might be. I poked around a little and there's actually a wikipedia article on this, so it might be worth taking a look at that, even if it doesn't have any specific numbers.
en.wikipedia.org