MB_LEN_MAX - maximum multibyte length of a character across all locales
The MB_LEN_MAX macro is the upper bound for the number of bytes needed to represent a single wide character, across all locales.
A constant integer >= 1.
ANSI C, POSIX.1
The entities MB_LEN_MAX and sizeof(wchar_t) are totally unrelated. In the GNU libc, MB_LEN_MAX is typically 6 while sizeof(wchar_t) is 4.