You should get 20 healthy plants from 20 germinated seeds.
Each plant must get at MINIMUM 6 hours of DIRECT sunlight per day plus indirect light to bring it to a total photoperiod of at least 18 hours of light and 6 hours of dark. With a solid-sided shed, and no supplemental lighting, I don't know how that is happening.
Furthermore, you need temperature control and ventilation, neither of which you have mentioned. How hot is it in the day, and how cold at night? Where does fresh air come from? Is your soil correct? Is it fresh soil? Have you fertilized your plants before 5 nodes?
REad the sticky in my signature and if you are doing things significantly different, we will try to troubleshoot them and get you back on the right track.