As a rough "rule of thumb" I use a teaspoon per litre (4-5 teaspoons per gallon) to get petillant carbonation. But a better method is to add sugar (or AJ or FAJC) to bring your SG up by 0.002-0.004 ( or more or less depending on what level of carbonation you want) to get the same result regardless of the quantity of cider you are dealing with. Obviously you will need a hydrometer to do this. This adds on top of whatever SG the cider is at now, so if you are above 1.000 and it still has a way to go, beware of bottle bombs.
See Jolicoeur's Table 15-3 for more information.