depends where you want your country,in the high arctic the sea is shallow so IIRC it took something around 1000 full six zed dump trucks to build endicot island ( i dont remember it very well but then i wasn't interested in what exxon was doing i was more concerned with the island being a hazard to navigation)so for a country i would have to say you would need several thousand dump trucks. and either enough military power or good will to keep the local sailors from gutting you like a fish.