A contact group can contain up to 3 million contacts. It is however recommended that you limit to a manageable number. For example, if sending to a contact group using a public channel, the max recipient in that group cannot be more than 1000 or the message will be rejected. When sending a message to recipients in a contact group using a private sender address or channel, the max number of recipients allowed should not be more than 100,000. Due to additional processing that is required when using contact groups to send messages, users should expect some delays in overall message processing.