Hi Nozomi,
This really depends from many things, like your nationality ad example.
Also it depends on the school as well. There are schools which are very strict on the bank statement and they require 20,000$ plus. In general for a student from America or Europe 10,000$ will be enough.
If you need more detailed information, specific to your case, you may contact me directly. Also please have a look to the website in my signature which I think is very informative.