Usually 10~15% are considered as "for redundancy". If you go further than that, you will likely lose continuity on your core business. The risk is just too much to take. Unless there's a good evidence that the business can sustain with 50% or whatever numbers, it'd better be safe.

It might true that some companies are extremely bloated and their competitors may be demonstrating a similar level of productivity with 30%~50% of people but different companies have different history and contexts; path dependency is the thing. For that level of productivity loss, the root problem is more likely deeply ingrained in the structure (especially management and decision making) so cutting more and more people usually won't help.