A holding tank in the RO
world is usually about a 4 gallon tank that sits under the sink to dispense RO
water to your sink from.
You want a container to hold RODI
water for your evaporation top off. You can use a used salt bucket if you buy those or anything. I use a BRUTE 45 gallon (round) container tha is sitting next to one sid eof my tank with a power head connected to a JBJ ATO to automatically add top off water was needed. Depending on tank size and the amount of daily evaporation you have, you may be able to get away with less. But, nothing beats the automation. I fill my 'resovoir tank' about every 3 weeks.