A multitude of sins in the Bible generally refers to a large number of sins that have been committed. In other words, it means that the person has sinned a lot. "For whoever keeps the whole law and yet stumbles at just one point is guilty of breaking all of it." (James 2:10) This quote from the Bible illustrates that even if someone has done well on most aspects of the law, but makes one mistake, they are still guilty of breaking all of it. In the same way, someone who has committed many sins is still guilty of breaking God's law, even if they haven't committed every sin possible.