Is there a way to redact a bank account number on multiple pages at one time?

I'm assuming you have Acrobat Pro. You can create an Action that executes the Search & Redact Text command (as well as a command to apply the redactions and then save the file), and run it on all of your files at once.

500 files is about the limit Acrobat can handle in a single process, though, from my experience, so you might want to do it in batches.