It's a large number, but it's hard to tell how large because the CPC keeps that information tightly under wraps. The official statement claimed just 200, but recently declassified diplomatic cables from the UK give an estimate of 10,000 dead (source). The original source was inside China's State Council. It's important to remember that the actions taken that day were far from universally supported even inside the party. There was a massive purge afterward of officials that were deemed to be sympathetic to the protestors.

Edit: This estimate likely has fog of war issues itself, though, since it was sent so shortly after the massacre. Other estimates are far lower, but still much higher than the official figures. The CPC does not want to admit the extent that it screwed up and killed its own citizens.