The scale actually goes up to '8 or greater'. The earthquake that caused the tsunami was a 9, it being one of the most violent quakes in recorded history.
Although 'greater than 8' is the highest value, you could quite easily say an earthquake was 10 on the Richter scale as it is a simple mathematical progression. The scale is logarithmic, so a force 6 is ten times higher than a 5, and a 7 is ten times a 6 (therefore a 7 is one hundred times a 5).