The short answer is you should only submit one game for the jam. If you finish before the deadline, use your extra time to add polish to help it stand out to the judges and the community.
The longer answer is you should not start building your game prior to the start of the jam, as that is when the theme will be announced. Your game must in some way adhere to this theme, so building your game beforehand, while technically is not against the rules, could be a waste as it likely will not fit the theme of the jam and require a completely new project or significant re-work.
