According to the Official Marvel Handbook's entry on Titan. "After*Thanos' attack, only a mere 100 Eternals were left. Before that assault, the population was of 35,000". Note that the destruction of Titan isn't bad because of how many he killed, it was who he killed. The Titans were known to be one of the most peaceful and technologically advanced societies in the known Galaxy. They were also incredibly powerful individuals. Destroying them was like a terrorist bombing a cancer research facility, or the Vatican, or some ancient Buddhist temple. His back story tends to have slight variations depending on who is writing it. Before he bombed Titan, we was off destroying planet after planet with his huge alien armada. He had wiped out many planets before he ever got to Titan. He even tried to fight Ego the Living Planet. After the whole destruction of Titan, he went on his several century long search for the cosmic cube, a device I'm sure he know has the knowledge to construct himself if he really wanted to.