The other news: The concurrent user issue is dependent on the web hosting and the database host. We do not have a recommendation for specific specs for RAM/Bandwidth/Processors/OS for the web server or for the database server. I would present your use case to your web host, or prospective web host, to find out their recommendations.
As an interesting data point, Basecamp launched their Hey email service, and one of the company founders tweeted a bit about the traffic and load and infrastructure. Here is one of the relevant twitter threads:
There is definitely some research to be done to support that many concurrent visitors. You won’t be able to handle that traffic with a $20/month shared hosting account.
Let us know if you have any other questions and if anyone else has specific recommendations, please feel free to share them. Thank you.