How many causes are there for losing power steering fluid?

There a a wide variety of possibilities: pump,rack,pressure hose, return hose, reservior are your main components. Fill the system up, have someone turn the wheel back and forth while you look for obvious leakage. If none, hold the wheel sharply for short bursts while you look. If still nothing, get some dye and add to it and check every 10 miles until you see it. If all else fails let a real mechanic fix it for you.