Gossamer Gear sells it's own high quality titanium Tite-Lite stakes. These tent stakes are the lightest ones we sell and a standard in all ultralighters toolkit. In General we like to have a combination of stakes in our kit, a few shorts, a few longs, and some "V" stakes for the ridge line and other key points.
For most tarps we recommend a minimum of 8, which gives you a stake at each of four corners, two guy lines, and two vestibule stakeouts. You may want to consider getting 12, which gives you extra stakes for fastening additional stakeouts or extras in case you leave one behind. This allows the use of all five stakeouts on the windward side of the tarp. For higher winds and added security, additional stakes would be an asset.