I think the meaning of the number varies from server to server, and depending on what the processes are doing. I've seen servers that consistently ran around 9 or 10, and held up fine. Wordie (which is on a VPS) basically stops functioning at 2. I once saw a box hit 220, to my astonishment. Must have been glowing white. It keeled over immediately after hitting that mark.
From Wikipedia: "In UNIX computing, the system load is a measure of the amount of work that a computer system is doing. The load average is the average system load over a period of time. It is conventionally given as three numbers that represent the system load during the last one, five, and fifteen minute periods."