Printf format string maximum width value (padding) %(??)d%n

Although it's not stated explicitly, the type of the field width is most likely an int. This is because if the field width is given as * then an int argument is expected.

Section 7.21.6.1p5 of the C standard regarding the fprintf function (any by extension printf) states the following regarding field witdh:

As noted above, a field width, or precision, or both, may be indicated by an asterisk. In this case, an int argument supplies the field width or precision. The arguments specifying field width, or precision, or both, shall appear (in that order) before the argument (if any) to be converted. A negative field width argument is taken as a - flag followed by a positive field width. A neg ative precision argument is taken as if the precision were omitted.

I tested this on CentOS 7 and Ubuntu 18, and the largest width I could specify was 2147483614 which is 33 less than the max value for a signed 32 bit int. If I use anything larger the first printf prints nothing and c remains 0.

Go generally speaking, the largest value you can expect for c would be INT_MAX, however the exact value will vary based on the implementation.