Kentico is able to handle a traffic of 5,000 concurrent users and to help support such a high volume of users, you can utilize Kentico's web farm module which takes care of synchronization between multiple servers.
Regarding the number of servers which are necessary, it is again hard to tell but I would not go with less than 2 servers. Additionally, what I would recommend is that you consider hosting your site in cloud (e.g. Azure or Amazon) due to easy options for scaling the project up and down. Adding or removing a server is a matter of a few clicks so you can experiment with how many servers provide you the best performance.