Bloomberg (via Teslarati) reports that the Amazon's stake is about 20%. According to the info that we received from Amazon, it’s part of $3.8 billion of “Other assets” – but not specific to Rivian as we previously thought.
"As of December 31, 2020 and September 30, 2021, equity investments not accounted for under the equity-method and without readily determinable fair values, including preferred stock of Rivian Automotive, Inc. representing an approximately 20% ownership interest, had a carrying value of $2.7 billion and $3.8 billion, and are recorded within “Other assets” on our consolidated balance sheets with adjustments recognized in “Other income (expense), net” on our consolidated statements of operations." - 10-Q SEC filling
The first $700 million was invested in early 2019. Capital is not the only thing that Amazon brings to Rivian, as the company is also a customer that placed an order for 100,000 all-electric Rivian Electric Delivery Vans (EDV).
Series production of EDV is scheduled to start in December and should reach 10,000 units in 2022. The remaining 90,000 should be delivered to Amazon within a few years - by the end of 2025, which indicates 30,000 per year.
Meanwhile, Rivian prepares for an IPO with a hope to see a valuation of $80 billion. The company so far raised more than $10.5 billion (including the $2.5 billion round in July) and besides Amazon, is backed also by Ford (probably more than 5% stake, considering over $820 million investment).
The first model, the R1T, entered production and is slowly ramping up, while the R1S and EDV should follow in December.