Ask HN: What are practical MongoDB load limits?

I have a client who asks me to make a sharded cluster of MongoDB instances to hold a really big and complex database. He insists on ability to serve 10M transactions per second, or 1M at least for the beginning. This brings up a question: is this really possible? Let's assume about 10% of transactions will be writes. What is the approximate number of machines in a cluster i will need? Say m4.4xlarge.

Please share your story - stories of extreme scaling of things are always nice to read :)