I had an original plan of just 2 44 gallon trash cans for water station but the LFS convinced me I needed more salt on hand in case I need to make a large water change so I got 3 instead which meant many trips to the hardware store.
Definitely best to have as much as possible on hand, My belief is if you can have about 50% on hand, that makes a huge difference. I'm doing 2 50 gallon containers, so with 50 gallons of RODI and 50 gallons of Saltwater, that puts me at 50% volume if I need to mix the 50 in RODI quickly.