20K is enough when the weather is good. We have multi-year droughts so it won’t be enough for the house when it’s bad. Cities were down to 140L per day so you can’t rely on municipal always.
With my 10K, all gardening and outside water was tank. A nursery with our heat uses a lot, daily watering.
I would err on the side of caution. We don’t use IBCs because they aren’t UV stabilised. Metal or UV plastic tanks are freely available.
A family of 4 with my larger nursery uses 31K every 3 months, it will get higher when El Nino arrives. I can’t convince kids to have short showers and we have an ultra low flow head.