size of screw, wall plug and drill bit for shelf upright with 7mm hole
I'd try my best to screw those into studs with the largest screw that will sit flush in the countersink. 

If you can't line it up with studs, get the largest anchor that will fit the countersink.

Something like: https://www.renodepot.com/en/hollow-wall-anchors-0484236 and NOT the plastic ones. This anchor will bend out in 4 directions behind the wall board and will distribute the weight on a larger surface area.

Of course, the one I linked does not have a countersunk screw so you'll want to pick some up as well unless you don't mind the head sticking out a little.

Again, the best would be to screw these directly to a stud.