Re: How many D-rings
My boat has two large D rings inside about 2 feet aft of the bow and two more pairs of small plastic D rings toward the middle. Sometimes I put a tie-down strap between the two large ones if I want to keep some gear up toward the bow (otherwise everything migrates to the stern).
Other than that, I don't tie stuff down with the D-rings. I've found my boat rides much better with more weight in the stern so I put the heavy stuff (anchor crate, fuel tank etc.) toward the back and it pretty much stays put. The only thing that gets tied down is the battery box, which is bungeed down with footman's loops on the deck.
As others said, take the boat out a few times and see what moves and whether you feel needs to be tied down before committing to any more D-rings.
However, if you have a bare wood or aluminum floor, one thing that might help is to get some kind of deck mat such as this:
http://www.northerntool.com/shop/tools/product_200306157_200306157. The non-slip surface helps to stop gear from sliding around (side to side at least) and it's safer to walk on when wet as well.
