I'd bet it's well into double digits relying on undefined behaviour, and over 50% relying on unspecified behaviour. It's hard to write interesting code in C that doesn't, where interesting means code that couldn't just as profitably be written in a different language with stronger memory safety.

Programmers have a mental model of many things - in particular, 2's complement signed numbers, and the idea that pointers are just numbers indexing memory - that are not guaranteed by the C spec. Much of the reason for programming in a lower level language is to take advantage of lower level machine characteristics, like tagged pointers, unsafe unions, structs with blobs of data appended to avoid an indirection, custom memory allocators, etc., but the C abstract machine doesn't necessarily give all the guarantees required, without a lot of care and attention to details.