Visit https://buttons.github.io/ first.

For my repo as an example: https://github.com/gazeldx/db_admin

  • Step 1: 
    <!-- Place this tag where you want the button to render. -->
<a class="github-button" href="https://github.com/gazeldx/db_admin" data-icon="octicon-star" data-show-count="true" aria-label="Star gazeldx/db_admin on GitHub">Star</a>
  • Step 2: 
    <!-- Place this tag in your head or just before your close body tag. -->
<script async defer src="https://buttons.github.io/buttons.js"></script>

FYI, I think buttons.js will call https://api.github.com/repos/gazeldx/db_admin.

The stars count is in the json of stargazers_count.