the number of users depends on the frequency, they are visiting the site, the modules installed, the content created - there is no easy calculation, but it shall be able to handle quite a number. Regarding scalablity, the first step would be to separate web server and SQL server, before installing a web farm (there is a dedicated guide on the Documentation page in Resources menu above). For high scaling web sites (like australian NFL and NBL, which are using DNN for their sites) you need some special optimization and partners, who are firm with this.