You can find your channel ID in https://www.youtube.com/account_advanced
Try it in preview in this link
Example
<script src="https://apis.google.com/js/platform.js"></script>
<div class="g-ytsubscribe" data-channelid="UC45pL-SL4KZwH9jB_VoAgXA" data-layout="full" data-theme="dark" data-count="default"></div>Preview