Messaging limits determine how many unique users or phone numbers your business can send messages. This includes new conversations as well as existing conversations with users. The messaging limit does not limit the number of messages your business can send, just the number of users you are trying to message. It also does not apply to messages sent in response to a user-initiated message within a 24-hour period.
Facebook has Tier levels that allows WABA to send messages. A business starts in Tier 1 when it registers its phone number (1K customers/day).
- Tier 1: Allows your business to send messages to 1K unique customers in a rolling 24-hour period.
- Tier 2: Allows your business to send messages to 10K unique customers in a rolling 24-hour period.
- Tier 3: Allows your business to send messages to 100K unique customers in a rolling 24-hour period.
- An unlimited number of business-initiated conversations in a rolling 24-hour period.
To increase the tier, there are ways that must be done, namely sending messages to a unique number.
For example, if yesterday sent messages to 800 unique number, today is 800, and tomorrow is 800, the day after tomorrow it will automatically go up to Tier 2 (10K customers/day) if the 2400 numbers above are all different (unique number).
For more information, you can refer to this link https://developers.facebook.com/docs/whatsapp/api/rate-limits/