The default limit is 30 API requests per second, and each request can only contain 1 call.
If you experience any difficulties or need to exceed this limit, please contact firstname.lastname@example.org
You can only have 1 active call to a number at a time also. If you need to send multiple TTS notifications to a number, you will need to wait for the first call to complete before starting the second. You can create logic to wait for the callback to be received from the Nexmo platform before submitting the second call.
Please note if you are using the new Voice API to send text-to-speech calls, using the Talk action, the maximum throughput is currently 2 request per second. If your throughput exceeds this maximum you will be rate-limited and receive the HTTP response error code of 429. This throughput for Voice API is fixed and can't be increased at the moment.