I voted four, more or less the norm for my main computer, a mid-tower PC. This consists of an external hard drive, UPS cord, mouse, and printer. So, the number is higher anytime I use other devices I own (MP3 player, several flash drives, digital camera {directly, or via card reader}, and webcam). I don't need a hub, since I never need to have so many devices connected at one time that they exceed the number of USB ports on the computer.
With my notebook PC, I use wireless mouse that uses a USB receiver. Sometimes, I use a USB keypad with it and, of course, sometimes connect a flash drive. Actually, the keypad can function as a two-port USB hub but it works at USB 1.0 speed as a hub.