The largest earthquakes recorded have been around 9.0 or slightly higher on the Richter scale. Technically, there is no limit to the potential magnitude. The most recent earthquake of that magnitude occurred in Japan in 2011.
The largest earthquake based on death toll belongs to the Tangshan earthquake of 1976 in China killing up to 655,000 people. The largest earthquake based on magnitude belongs to the 1960 Chilean earthquake in 1960, which reached 9.5.