When a plant of most kinds "hits" the top of something while growing, it changes its tactics of growing and starts to grow the bottom branches of the plant upwards to get as much light as possible at all times. If that does not work, the plant will start to grow horizontally. All of this is just a survival tactic that the plant uses to survive. When you are growing indoors, you cannot harness the power of the sun (unfortunately), you use the best that you can get at the time (usually HID and MH) These lights do not penetrate the foliage of the plant as well as the sun does, and even the sun cannot fully penetrate a large plant. This usually cause's the top "half" (depending on size) of the plant to be larger, more dense buds, than the bottom of the plant. This is why the scrog method was invented. It utilizes the plants own survival instincts and helps stop light waste to parts of the plant that would produce less than quality smoke. This is achieved by allowing your plant to fill a portion of your screen above the plant. usually people will let the plant grow above the screen for a couple of days, then pull it back under the screen and direct it wherever the screen needs to be filled. Once you have filled most of your screen, you start to flower your plant. The plant will go through a stretching period and then stop, and if done correctly, you will have a nice even canopy of flowering plant above your screen. Now, when filling your screen, you do not want to fill the entire screen. You want to keep the foliage under the screen until you switch to flower. When you switch to flower you must train your plant during its stretch until its stretch is over. Once the plant begins to flower you will notice that the light penetrates the entire canopy evenly, making all of the bud on the plant "top" plant quality. There is more than one way to skin a cat, so this method is not the end all be all. Do some research, look at some grow journals, and ask some questions, then make your own variation of the method so that it works for you.
P.S. A good rule of thumb is 1 plant per square foot of growing space, this is true for SOG, SCRoG, and most other growing methods. So, if you were to grow all 20 plants, you would need 20 square feet of growing space, plus the lighting need to light that space.
