Is there a limit to the number of requests a user can send?

Messaging Limits: Messaging limits determine how many unique users your business can send messages to on a daily basis. 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.

Tier 1: This allows your business to send messages to 1K unique customers in a rolling 24-hour period.
Tier 2: This allows your business to send messages to 10K unique customers in a rolling 24-hour period.
Tier 3: This allows your business to send messages to 100K unique customers in a rolling 24-hour period.

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For more details, please visit:

[https://developers.facebook.com/docs/whatsapp/api/rate-limits#quality-rating-and-messaging-limits]