@ksyt Thank you for your prompt and accurate response.
How to count the number of messages The number of messages is counted by the number of people to whom the message was sent. For example, even if you send a push message with four message objects specified in a single request to a chat room containing five people, the number of messages counted is five. The number of message objects specified in a single request doesn't affect the number of messages sent.
Also, if you send a message to a user ID that has blocked the LINE Official Account, a user ID that doesn't exist, or to any other users that won't actually receive your message, it won't be counted as one message.