How many vents and returns and where should they go??

So as it stands, I have two heat vents and one return for the whole downstairs. The return did go down a wall to the floor but I have removed that wall. How many vents and returns should I have to sufficiently cover this area and where should they go? Should I keep them all in the soffit? There is only a couple areas That I can bring the vents or returns to the floor due to the foundations sill. Maybe just add one heat vent for the bathroom? (The room I am standing in when I took this picture will be the bathroom. Next to it, the kitchen then an open studio type room.) Any suggestions are appreciated Attachment 38828 (http://www.doityourself.com/forum/attachments/basements-attics-crawl-spaces/38828-how-many-vents-returns-where-should-they-go-20140924_174756.jpg)