How much is needed for bank ceritificate?

The money you sould have on your bank account should be USD 100 per day. The longer you'll stay in China, the more you should have. Of course, if you have more than it, it definitely will be helpful for you to get the visa successfully.
Yes, one can re-apply for the visa right away after getting denied. But you'd better conclude why you get denied and re-apply with the solution.