How many RPUs does it take in MongoDB aggregate, if multiple documents in the collection loopup to a single document in another collection?

Working with Data
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3

I don’t believe i’ll be able to advise how much RPU this scenario would generate but I would recommend reading over my response in the following post for more information that may help : Serverless RPU model - #3 by Jason_Tran

In saying so, there’s also a feedback for a Serverless RPU estimator post which you can vote on which I believe would help with your use case noted here.

Regards,
Jason