There are no hardware requirements for running Kentico CMS. However there are some software requirements. Please see here for details: System requirements
So in other words, the minimum hardware required is that which will allow you to install and run required software and components.
However, recommended hardware configuration always depends on the particular project, its configuration (single server, web farm, load balancing), its size, expected traffic, the nature of the content, number of visitors, and so on.
You can use our performance reports as guidance:
Kentico CMS 6.0 Performance report
Kentico CMS 5.5 Performance report
Kentico CMS 5.0 Performance report