I think you made a mistake in the title "multiple fax ...", coz your question is about "one or more documents at a time..."

To send multiple documents in a fax, just append more attachments to the FormData. Here is an example in Node JS using the RingCentral JS SDK.

async function send_fax() {

  try{



    var endpoint = "/restapi/v1.0/account/~/extension/~/fax"



    var FormData = require('form-data');

    formData = new FormData();

    var bodyParams = {

      to: [{'phoneNumber': RECIPIENT}],

      faxResolution: 'High',

      coverPageText: "This is a demo Fax page from Node JS"

    }



    formData.append('json', new Buffer.from(JSON.stringify(bodyParams)), {

        contentType: 'application/json'

        });



    formData.append('attachment', require('fs').createReadStream('test1.jpg'));

    formData.append('attachment', require('fs').createReadStream('test2.png'));

    formData.append('attachment', require('fs').createReadStream('test3.png'));



    var resp = await platform.post(endpoint, formData)

    var jsonObj = await resp.json()

    console.log("FAX sent. Current status: " + jsonObj.messageStatus + "/" + jsonObj.id)

    checkFaxStatus(jsonObj.id)

  }catch(e){

    console.log(e.messsage)

  }

}



async function checkFaxStatus(id){ 

  try{

    var resp = await platform.get(`/restapi/v1.0/account/~/extension/~/message-store/${id}`) 

    var jsonObj = await resp.json()

    console.log(jsonObj)

    if (jsonObj.messageStatus == 'Queued'){

      setTimeout(function () {

        checkFaxStatus(id)

      }, 5000)

    }

  }catch(e){

    console.log(e.message)

  }

}