
Diamond
Better than a standard tent peg, because the thread grips along its whole length rather than relying on friction down one side. In loose, dry sand nothing holds brilliantly, so angle them away from the pull, and in really soft ground weight the mat corners as well.
The bit is in the kit, so a cordless drill makes it a few seconds per screw. By hand with a driver works on soft ground and is quieter early in the morning, but on hard-packed gravel you will be very glad of the drill.
For a corners-and-midpoints job on a typical mat, yes. On a long mat in windy country you will want more along the exposed side, because that is the edge that lifts. Plenty of people run a kit per mat and keep a spare handful in the van.