This reads the Link HTTP header and counts the pages of commits (the contributors and search API's do not give accurate commit counts).

github-commits.js

const fetch = require('node-fetch');
const parse = require('parse-link-header');

async function githubCommits(owner, repository) {
  const response = await fetch(`https://api.github.com/repos/${owner}/${repository}/commits`);

  let link;
  if (response.headers) {
    for (let pair of response.headers.entries()) {
      if (pair[0] === 'link') {
        link = parse(pair[1]);
      }
    }
  }

  if (!link) {
    const commits = await response.json();
    return commits.length;
  }

  let commits = (link.last.page - 1) * 30;
  const lastPageResponse = await fetch(link.last.url);
  const lastPageCommits = await lastPageResponse.json();
  return commits + lastPageCommits.length;
}

console.log(githubCommits('rails', 'rails').then(commits => console.log(commits)));