People often ask us whether mojoPortal CMS can handle x amount of traffic. Unfortunately, this is not an easy question to answer and anyone who gives you easy answers to this kind of question probably should not be believed. In general, you don't need to worry about capacity planning for your mojoPortal web site unless you are expecting really huge amounts of traffic. Generally, even if you are headed for that level of success it will not happen over night and you can gradually scale up from shared hosting to a dedicated server. A sufficiently beefy single server should be able to handle quite a lot of traffic. The next step would be to move the database to a dedicated server. Usually the database server will be a bottle neck before the web server so you may need a database cluster before you need a web cluster. Beyond that you can look at scale out strategies like web farms.
mojoPortal content management system is designed to perform well and to make efficient use of available resources. We do load testing on a regular basis between each major release to make sure I'm not losing performance over time, though our testing is not exhaustive of all features and all scenarios. We've also done load testing comparisons of mojoPortal vs other .NET apps and mojoPortal compares favorably.
As far as specific numbers of user and requests of course that depends on hardware. Capacity planning requires a lot of effort. There is a very good article "How To Perform Capacity Planning" which outlines the steps involved with coming up with hard number predictions. As you can see it's a lot of work to really do it right for all features. Because of the various scenarios which could exist, we don't make any bold claims. We can say that we have built and supported sites with hundreds of thousands of page views per month on moderate hardware with no performance issues.