So the practice is different to the theory, yes you can run that many products in Magento and it's the best solution below Hybris or Websphere or Demandware, but you have to run it on clusters and not any clusters, special performance clusters which the consultants designed in-house.
These volumes (or close to) have been on both 1.9x and 2.4x, the problem with 2.4x is due to the new way of doing everything it's more susceptible, meaning day to day it works better than 1.9x but when it breaks your whole site can go down, anyway unless you are willing to spend $millions on enterprise solutions Magento2 will be your best option.
As a note be careful of custom code and non-standard extensions, the lack of testing and bugs with that size store will result in no end of problems from security to stability, plus if you're running community they removed the read/write database instance separation, there's a workaround which we use but it's part of the cluster.
On top of that it was simpler for us because we have auto load code that creates and maintains the products and image generation via a drag/drop product manager, it was designed to load 100,000s products per day and process 10s millions to check for updates, this is something else you will need to consider, plus the fact Magento only has two-tier currency (international sales), you need three-tier but again we have special code that does this by rewriting the pricing engine.
In theory it's simple, and if you have all the above it is, but for most it won't be and would take you 6-9mths to launch, we do these in around 6-9wks but it's for in-house sites because the developers have experience from multi-national companies who run millions and 10s million product websites, anyone with only 10,000s product launch will just not have the experience.
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