Hi Nick,
First of all, please read this document for detailed information about using SMS.
Recently, we introduce the Toll Free SMS feature and it is meant for A2P (Application to Person) SMS. Right now, the sending rate for Toll Free SMS is also 40 messages per minute. But we are going to increase the rate in the near future. We'll also support the High Volume SMS which enhances our Toll-Free SMS capability to send 3 messages / second or 180 messages / minute. This is over a 4x increase of our current rate of 40 messages / minute. You can click here to sign up for early access.
Back to your question, you have to send each message per post request because if you specify multiple recipient in the "to" array, the system will send as a group message.
If you choose to use the Toll Free SMS now, login your account at https://service.ringcentral.com and add a Toll Free number.
Then you can use this app to send SMS messages to multiple recipients.
If you want to implement such an app yourself, you can clone the project from this github repo and change it as you want to.
