Using a single WhatsApp Business API call, you can send up to 20 Messages per second. Burst Capacity (where the throughput temporarily exceeds the sustained throughput rating) is available and messages which hit that burst capacity will be queued for send. This really only becomes an issue when you have particularly time sensitive content, for example, OTP.
Sinch can offer higher throughput APIs at additional cost. Please contact your Sinch Account Manager for more information.