How to use GitHub V3 API to get commit count for a repo?

If you're looking for the total number of commits in the default branch, you might consider a different approach.

Use the Repo Contributors API to fetch a list of all contributors:

https://developer.github.com/v3/repos/#list-contributors

Each item in the list will contain a contributions field which tells you how many commits the user authored in the default branch. Sum those fields across all contributors and you should get the total number of commits in the default branch.

The list of contributors if often much shorter than the list of commits, so it should take fewer requests to compute the total number of commits in the default branch.

You can consider using GraphQL API v4 to perform commit count for multiple repositories at the same times using aliases. The following will fetch commit count for all branches of 3 distinct repositories (up to 100 branches per repo) :

{
  gson: repository(owner: "google", name: "gson") {
    ...RepoFragment
  }
  martian: repository(owner: "google", name: "martian") {
    ...RepoFragment
  }
  keyboard: repository(owner: "jasonrudolph", name: "keyboard") {
    ...RepoFragment
  }
}

fragment RepoFragment on Repository {
  name
  refs(first: 100, refPrefix: "refs/heads/") {
    edges {
      node {
        name
        target {
          ... on Commit {
            id
            history(first: 0) {
              totalCount
            }
          }
        }
      }
    }
  }
}

Try it in the explorer

RepoFragment is a fragment which helps to avoid the duplicate query fields for each of those repo

If you only need commit count on the default branch, it's more straightforward :

{
  gson: repository(owner: "google", name: "gson") {
    ...RepoFragment
  }
  martian: repository(owner: "google", name: "martian") {
    ...RepoFragment
  }
  keyboard: repository(owner: "jasonrudolph", name: "keyboard") {
    ...RepoFragment
  }
}

fragment RepoFragment on Repository {
  name
  defaultBranchRef {
    name
    target {
      ... on Commit {
        id
        history(first: 0) {
          totalCount
        }
      }
    }
  }
}

Try it in the explorer