so if i had 5 plants yielding 3oz per plant would 20 plants yield 3oz per plant still........
Not under the same light.
you can get 20 clones, and "lollypop" them. That means cutting of all of the side shoots and putting them into flower RIGHT away. Each one would probably yeild about .5 oz's. Some a little more, some a little less.
In 8-9 weeks when they finish flowering you would likely get in the neighborhood of 10oz's total.
If you vegged 5 plants for about 4-5 weeks first you could expect about the same yeild. It will have just taken you an extra 4-5 weeks to make it happen. Still 8-9 weeks of flowering (indicas).
Again, these numbers are just to show you the difference and should not be used as your guidelines. First grow, expect less.
You are probably best off growing about 5-6 plants and vegging them about a month and going through the entire process.