Varies I would think on area, soil, growing conditions. Would be more than 25 by a long shot in Wisconsin where I am. We have big plants, but 30 or more years old. If I were in your shoes, and I think you are doing well starting out I would keep putting seed in the ground and try to wait a couple more years than you plan. Dig 9 roots from different areas on your land and weigh on a reloader scale, 1 oz. is 438 grains. If for instance your 9 roots weigh 6 oz. green, that is approx. 2 oz. dry. 8 times 9 roots is 72 plants. Replant the roots.
I found 45 roots that were 15 to 40 years old, two and a quarter pounds green. 36 oz. so that is about 3/4 oz average. Very dry by us, everything in the woods is suffering.
Hope this might help you, I think you are on the right track. Never stop the planting, even if you cut down. You should have enough of your own as to not to have to buy.