How much rack space you require is obviously determined by the number of servers you'll be using. Keep in mind, though, that any network appliances such as switches and routers also take up space.
In planning for future expansion, you may wish to allow a certain amount of free rack space beyond your immediate requirements. If you buy only 5U in a shared rack, for example, it is almost inevitable that space either side of you will be sold by your ISP more or less right away and that future equipment may have to be cited elsewhere as a result.
Assessing your server requirements is a little more tricky than figuring out how much bandwidth you'll need. After all, every server is different and is capable of handling highly different numbers of simultaneous users.
You can be sure, though, that you will need at least one Web server and at least one database server to host your application. On much smaller sites you can probably get away with using a single shared server, but both MySQL and PostgreSQL are not friendly to other processes on the same machine. Your site will suffer as a result.
The real question is, then, how many of each? Only guesswork or load testing will establish this, but if you do find yourself needing more than one server, the following section may help you.