I have a 21k gallon pool how much algaecide and chlorine should I put in for closing thank you

I have an 11k pool n I put in 2oz of algaecide so I would say 3-4oz of algaecide for yours. As far as chlorine goes I put in half a cup for maintenance purposes. For your pool almost double.

If the pool is crystal clear and your water is balanced - pH 7.2 - 7.6, total alkalinity 80 - 150, and calcium hardness 175 - 350, I recommend 2 gallons of shock and a winterizing kit which contains a winterizer for stain and scale prevention, and a long term winterizing algaecide. Depending upon the fall/winter/spring season, using 5% algaecides or 10% algaecide that come in the gallon bottles - they may not keep the pool clear through out the long time the pools are closed. If you chose to use those, I would recommend getting a high chlorine level and then using 1 gallon if using 5% algaecide and 1/2 gallon if using 10% algaecide. If we get a warm early spring, I would recommend adding additional algaecide in the spring - March or April depending upon the weather. This assumes you are using a solid cover on the pool, not a mesh cover or a solid cover with a mesh drain panel. Hope this helps.