Where can I find a list of JSHint numeric error codes?

The best place to look for things like that is the source (which is available on GitHub). The file you’re looking for is messages.js (versions: current release (2.9.5, 2017-06-22), master branch, 2.1.4 (source for the code below)):

var warnings = {
    W001: "'hasOwnProperty' is a really bad name.",
    W002: "Value of '{a}' may be overwritten in IE 8 and earlier.",
    W003: "'{a}' was used before it was defined.",
    W004: "'{a}' is already defined.",
    // ...
};