App Script for retrieving the concurrent viewer count of a YouTube live stream by channel ID.

Code.gs

function doGet(e) {
  var channelId=e.parameter["channelId"];
  
  var results=YouTube.Search.list("snippet", {"channelId":channelId, "type":"video", "eventType":"live"})
  if(results.items.length!=0) {
    var broadcastResults=YouTube.Videos.list("liveStreamingDetails", {"id":results.items.pop().id.videoId});
    var output={"concurrentViewers":broadcastResults.items.pop().liveStreamingDetails.concurrentViewers};
    return ContentService.createTextOutput(JSON.stringify(output)).setMimeType(ContentService.MimeType.JSON);
  }
}