While how much money you bring to Japan is entirely up to you, you should keep in mind that most JET's in Oita don't receive their first pay until three to four weeks after coming to Japan. Of 27 JET's surveyed in 2017, twelve received their first pay after three weeks and eleven after four weeks.
You'll use your money on a variety of things including:
Apartment deposit/key money/set up fees (for some JET's this is very little for others it can be quite expensive)
Furnishing your apartment/house (sometimes you have the option to buy items from your predecessor)
Car (some JETs need to drive for work, others choose to for lifestyle reasons)
Bike (some JETs need a bike to get to work, others choose to for lifestyle reasons)
Enkai (drinking parties)/other school events/functions
Mobile phone (if your phone is unlocked you may be able to get a sim card plan, otherwise it is possible to buy phones outright or as part of a phone plan)
Work clothes
Food
Travel (there are a some public holidays over summer which are an excellent chance to explore and travel)
This information was gathered in 2017 from 27 JET's who participated in an information and advice gathering for new JET's
ALL YOU NEED TO KNOW ABOuT MONEY MATTERS IN JAPAN A.K.A. JET ORIENTATION MONEY AND SENPAI ADVICE PRESENTATION