WhatsApp Cloud API price model is conversation based.
Each conversation either by business initiated or customer initiated will be counted.
So when a user sends you a message it starts a conversation. And in 24 hours window you can send & receive unlimited message which will be counted 1 conversation. This is user initiated conversation.
In other hand, when a business sends template message to user it also starts a conversation. And in 24 hours window you can send & receive unlimited message which will be counted 1 conversation.This is business initiated conversation.
And you have 1000 free conversation each month.
Here is the details of WhatsApp Cloud API pricing mode : https://developers.facebook.com/docs/whatsapp/pricing/

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