If it's going to be a problem getting paint in the future, I would buy twice as much as I needed. For starters, it always seems to take a little more than you think in the first place and you never know, you might accidentally spill some of it or ruin some of it in some way. Then you have to think about possible repairs in the future. I would buy at least 2 gallons of base with the corresponding amount of proper reducer and also 2 gallons of clear with the corresponding amounts of reducer and hardener. This should leave you some for future use. We use PPG DBC base which doesn't use an activated reducer so we can simply pour the unused amount back into the gallon can. When you finish painting the car, store the unused paint in as small a can as possible, label it, tape it shut, and store it in a spot where the temperature stays at a reasonable temp. If you ever have a fender bender, you will have the correct color when you need it.