As many as it takes for the shot...which is a number between one and six most of the time. Like Big Mike pointed out, some glamour photogs like Playboy's longtime contributor Arney Freitag would use 10,15,or even 20 lights (mostly continuous light sources too, and not flash units). If you look at a main light, a fill light, a hair light, two background lights, and possibly a separation light, that is six lights. Six lights is really quite common on many people set-ups.
Another question you might want to consider is, "What *kind* of light modifiers are useful?" With the answer being umbrella, soft box, parabolic reflector, diffusion panels, reflectors, grids, snoots, gels, and so on.