We do not recommend sending A2P SMS using a US Long Code SMS. Instead, you should procure a Short Code for such use cases.
Incase of a legitimate use case using US Long Codes, we encourage you to send less than 200 messages per day from each U.S. Long Number you purchased from Plivo. To avoid the risk of getting flagged as spam by carriers, it is desirable to send the SMS using multiple Plivo numbers as your sender id.
For example, to send 6000 SMS/day, one could purchase 30 Phone numbers and balance the sent SMS out using different phone numbers as the sender id.