Depends on the style you want, the carbonation level you want, the temperature at which you're bottling . . . several factors. Use a calculator like this one
I can tell you that for my 5 gallon batches, at 70F or so, I like 2.3-2.5 volumes of CO2 in my pale ales. I put 4oz / 110g of cane sugar in 500mL of water, boil it for a few minutes, cool it, then mix it in with my beer in the bottling bucket. I've been very happy with the results.