RingCentral high volume SMS throughput is up to 3 msg per second. That makes about 250K messages per day.

There is no max number of messages for high volume SMS API. However, there is a max size of the /batches API payload to post a batch and that is 50MB. So the numbers of messages per API call will depend on the message length and the way you send the message (broadcast or personalized campaign)

Read these articles for more info about HV SMS.

https://medium.com/ringcentral-developers/sms-for-business-application-to-person-sms-c11d826ee447

https://medium.com/ringcentral-developers/high-volume-sms-and-php-d849dc49dc16

https://medium.com/ringcentral-developers/how-to-achieve-great-sms-performance-in-non-consumer-text-messaging-380574a6a78d

https://developers.ringcentral.com/guide/messaging/sms/high-volume