You could take low-end instance specifications and standard industry stacks and extrapolate forward how many instances they might need to maintain at a maximum, but those numbers are going to be off.
Are they running 400 low-end front-end instances across the globe? Probably, (plus the 40 or so other services they claim to need, multiplied by region count at a minimum) and that would actually be well below realistic and reasonable for a company like Amazon. You can take a bunch of regional instances that handle roughly 200 req/s and make that work.
There's also a common misnomer that Amazon.com is somehow just this one giant app running on a set of servers, which isn't remotely how it's actually deployed, and that's before we spend time arguing whether a team's instances even count as "primary e-commerce front-end stack" or not. :P
Not to mention all the data center logistics
The number probably changes all the time based on load. They'll never release these details because it's a competitive advantage to have the "how popular are they in $place at $time-of-day" data private.
When they do share numbers, it'll always be the most flattering and devoid of any context beyond the "wow" factor.