Machines go down. It's a fact of life. They may need to be rebooted for some reason, or they may have a hardware failure, or a power outage. So if I wanted to deploy a website with a server backed by a SQL database, putting the whole thing on one server wouldn't be good enough. It obviously needs at least two servers, so that if one goes down, the other can pick up the slack until the first comes back up.
Of course, if I have the server software on two machines, either one of which could go down, I can't place the database on either of those two machines, because it could go down. So the database needs its own server. But that server can go down, so I need a backup database server and some sort of replication system to keep it in sync so the main can fail-over to it.
So far, that's a bare minimum of 4 machines to keep one website running with a reasonable chance of no downtime. (Assuming no catastrophic events take place that take down both front-end servers at once or both DB servers at once, and no hacks, DDOS attacks, etc.
Am I missing any other factors, or should I consider 4 servers to be the minimum for running a website with a goal of continuing operation without downtime even when a server goes down?