need help with charging system!!!!

18-20 volts is too high. 11.5 volts is too low.

Why did you replace the alternator, voltage regulator, and battery in the first place? What stopped working, when, and how? Just tossing parts at a broken car is an unsatisfactory, expensive way to fix them. What brand and model of voltage regulator did you put in? Whose alternator? There's a lot of junk on the market these days; it's relatively hard to buy good quality parts.

Info on testing and repairing charging systems is here, here, and here. As soon as you can, get hold of the three books described in this thread.