Using the type of grow system you are speaking of, (I already do this, just not at this size), you need to know the production of a give species on the average.

For example I don't let any of my stuff get over 4 feet tall. I have indica and sativia genotypes, some strong in each category. I still get about 1.5-2 ounces of dried bud, never weigh out the trim, which I use for the beginnings of cannabutter.

So, at 20 plants in the flower room, if you want to continue taking out a certain amount every week, say 2 per week, then that gets you4 oz a week.

The point is, you have so many variables in plant production, size, type, veg time, etc, no one can honestly answer that question with the information provided.