Pretty hard to over do it on perlite and any amount will help so I'd say as much as you're willing to pay for
A bag a piece would be good, two woul dbe great but half a bag a piece would be better than none. If your plant's roots aren't already established throughout the soil you can just mix it in by hand all round the plant. Or you can remove soil while not disturbing the plants roots, amend it into the soil and replace.
Have you thought about amending anything else into the soil? Even if you're using a bottled line or any other chem fert I would still recommend ammend the soil with a few pounds of kelp meal and alfalfa meal. Both contain natural plant growth regulators that have been found to be beneficial.