WhatsApp Business Message Limit

The maximum tested outbound message rate is 20 messages per second.

WhatsApp imposes a capacity rate limit on each API endpoint, which returns the appropriate HTTPS error codes (e.g., 429, 503, etc.) depending on the state of the system.
WhatsApp supports two forms of capacity rate-limiting:

• Request rate-limiting: It is the throttling of incoming requests on an API endpoint. When the number of requests per second limit is reached, the endpoint will start returning a 429 error code.
• Concurrent rate-limiting: Concurrent rate limiting is the throttling of incoming requests when the Core app itself is under heavy load (e.g., job queue size grew too large). At this point, the endpoint will start returning a 503 error.

In such a scenario, you should halt any further requests and slow your API requests' pace. If you start seeing many capacity rate limit errors, it would be advisable to build a queue on your end to throttle the requests.

Messaging limits determine how many users your business messages in one day. This includes both new conversations and existing conversations with users. The messaging limit is not a limit on the number of messages your business can send. It just limits the number of users you are trying to message.

• 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.

A business’ phone number will be upgraded to the next tier if:

• Quality rating is not low
• If the sum of users it sends notifications to is twice its current messaging limit within seven days.
  Once the business reaches this threshold, it will be moved to the next tier.

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Note:

The upgrade will take 48 hours or more.

Refer this page, for more information on Capacity, Quality Rating, and Messaging Limits.