SMS API Group?

Very likely you login every time you call send an SMS message, and the /auth endpoint has API call limit at 5 calls per 60 secs.

To avoid that, login once then call the SMS endpoint repeatedly. E.g.

var SDK = require('ringcentral')
var rcsdk = new SDK({
      server: RINGCENTRAL_SERVER,
      appKey: RINGCENTRAL_CLIENTID,
      appSecret: RINGCENTRAL_CLIENTSECRET
  });
var platform = rcsdk.platform();
platform.login({
      username: RINGCENTRAL_USERNAME,
      password: RINGCENTRAL_PASSWORD,
      extension: RINGCENTRAL_EXTENSION
      })
      .then(function(resp) {
        for (var i=0; i<40; i++)
          send_sms()
      });

function send_sms(){
    platform.post('/account/~/extension/~/sms', {
             from: {'phoneNumber': RINGCENTRAL_USERNAME},
             to: [{'phoneNumber': "1234567890"}],
             text: 'Hello World from Node JS'
        })
        .then(function (resp) {
          var jsonObj = resp.response().headers
          console.log(jsonObj['_headers']['x-rate-limit-limit'][0])
          console.log(jsonObj['_headers']['x-rate-limit-remaining'][0])
          console.log(resp.text())
          console.log("SMS sent. Message status: " + resp.json().messageStatus)
        })
        .catch(function(e){
          console.log(e.message)
        });
}

More about API Rate Limit

It is not true that if you want to send more than 40 messages, you should use multiple numbers. Read this blog for detailed info.