This option will be manage from Other Settings > WhatsApp configuration > Gear Icon > find Mass Message Configuration
Batch size allows the user to send mass messages in a batch of messages to avoid Spamming.
Example : If you wish to send 500 messages and if you set a batch size 15 and Time Interval to 15 minutes. Then it will send 15 messages in each batch based on the Time Interval calculated below.
Time to send 500 messages = (500 messages / 15 Batch size) * 15 minutes = 500 minutes = 8 Hours Approximately.
Random : Random selection dynamically select any number from 1 to 25 as a batch size when sending message to customer, It means if there is 500 messages to send and Batch Size – Random than it will send 5 messages in first Batch, 8 messages in a second Batch and so on..