Scalability means how many users your site can handle. Scalability often comes at the expense of performance, especially when distributing a website over multiple servers. From our experience, it is much better to use a single, powerful server than to use multiple medium-sized servers.
Up-Time and Redundancy
For commercially interesting projects, we recommend setting up a 'hot spare'. This means we duplicate the entire production system. In case of a primary system failure, this hot spare will kick in to serve your customers, while technicians step in to repair the production site. This is often the only way to guarantee 100% up-time. (Facebook, Amazon, Google have thousands of such redundant servers.) A cloud-based alternative, such as CloudFlare, offers a lower cost solution, but only for static pages, not for dynamic content.