about 1.5 tablespoons for 6 oz of uncooked rice (as per the recipe at this link:
http://www.bento.com/trt-sushirice.html ) or pasted below:
Sushi Rice (Sushi-Meshi)
Ingredients:
175g / 6 oz uncooked, matured short-grain rice
225ml / 8 fl oz cold water
2.5- to 5-cm /1- to 2-in strip dried kelp, wiped clean
For sushi vinegar:
1 1/2 tablespoons rice vinegar
1 tablespoon caster sugar [superfine granulated sugar]
1/2 teaspoon salt
This recipe will make one quantity of rice, which is sufficient for
2 uncut large rolls (futomaki-zushi)
or 4 uncut small rolls (hosomaki-zushi)
or 16 finger sushi (nigiri-zushi)
This is the basic technique for producing the glutinous, vinegar-flavoured rice that forms the basis of all types of sushi. Matured, Japanese or Californian short-grain rice is essential. To vary the quantity of cooked rice, remember that the ratio of uncooked rice to water should be 1 part rice to 1 1/4 parts water.
1. Start by washing the rice thoroughly until the water comes clear. Let the rice drain for 30 to 60 minutes. This will allow the grains to absorb moisture and start to swell.
2. Put the rice, water and kelp in a pan with a tight-fitting lid. Bring the mixture to the boil over a medium heat, removing the kelp just before the boiling point. Cover the pan and simmer for about 10 minutes. (Simmering time will vary depending on the quantity of rice.) Resist the temptation to lift the lid while the rice is cooking.
3. Turn off the heat, remove the lid and cover the pan with a teatowel. Leave to cool for 10 minutes. Meanwhile, mix together the ingredients for sushi vinegar in a pan. Heat until the sugar dissolves, then remove from the heat and pour the sushi vinegar into a cool bowl. To stop the vinegar distilling off, sit the bowl in cold water to speed cooling.
4. Using a wooden rice paddle, spoon the rice into a rice tub or bowl. Spread it out evenly, then run the paddle briefly through the rice cutting it first from side to side, then from top to bottom.
5. Continue cutting - never mashing or stirring - the rice, adding the sushi vinegar a little at a time. At the same time, ask someone to fan the rice to cool it. It should take about 10 minutes to mix in the sushi vinegar thoroughly and bring the rice to room temperature.