C doesn't even define them, other setting a minimum. There have been C implementations where char was 64 bits.

As far as the documentation for the C langauge where they speculate that it's not defined, just given a minimum bound: here you go: Their implementation-defined values shall be equal or greater in magnitude (absolute value) to those shown, with the same sign (C99 spec, section 5.2.4.2.1)

It also allows sign-magnitude integers (complement 1) in addition to complement 2 (C99 spec, section 6.2.6.2)

It also doesn't define the number of bits in a byte (C99 spec, section 3.6 p2)