I need to light a pyre ship for the fremennik diary and I'd like to know how many kills it took other players to get a chewed bone.

Getting a chewed bone from a Mithril Dragon is quite rare, I'd have to say 1/50 at least. You can, and should use mangled bones which are easier to obtain.

It's a shame PvP crashed the price of a DFH I'm quite enjoying killing these. And yeah, a mangled bone would be fine as well.

Mangled bones are a 100% drop from confused barbarians, which you can find in that same cave.

Oh, thanks, you just saved me lots of time... it should really say that in the fremennik diary or pyre ship (firemaking) guides.