UPDATE ClassCraft reports one Meteor server handling 6400 concurrent users. Read more in the Scalability section of my Why Meteor article.
See also the forum post
How Many Simultaneous Users Does the Biggest Current Meteor App Support?
Here’s a benchmark not exactly about concurrent users, but rather about setting up subscriptions (arguably a more stressful test).
The test was run on the cheapest Digital Ocean servers (512MB RAM, $5/month), and the results were rather impressive:
For all the above scenarios, server response time is less than 8ms.
Essentially one commodity machine can handle 2500 Meteor subscriptions per minute and respond within 8 miliseconds. Horizontal scaling takes less than 5% overhead.
You can read more about the test at Meteor Cluster Performance Test: Impressive Results.