There are no specific quantities to this, just a general rule of thumb: allow about 5 to 6 new potatoes per person, use enough olive oil to coat them, and use salt and pepper to taste.
Boil the new potatoes in their skins until just tender. Drain. Place them whole in a casserole dish, and mix in with them the olive oil, the garlic, sea salt as well as some black pepper.
Roast uncovered in the oven until the potatoes are crisp.
Notes
You can cook them at a lower temperature if you need to because something else is also in the oven, but try in the final 10 minutes of cooking to give them a real blast of heat.