Complex sentence: He is the first American, and 12th person overall, to summit all fourteen mountains over 8000 meters (collectively known as the eight-thousanders), and the sixth climber to do it without bottled oxygen.
Split into simpler sentences: He is the first American, and 12th person overall, to summit all fourteen mountains over 8000 meters (collectively known as the eight-thousanders). He's the sixth climber to do it without bottled oxygen.