The amount of aspirin you can give your dog depends on the dogs weight.
The general guideline and recommended safe dosage for giving dogs aspirin is between 5-10mg of aspirin per pound of dogs weight.
So if your dog weighs 10 lbs you would give the dog around 25 mg of aspirin and usually 40 to 50 mg is more than enough to give larger dogs.
If you give the dog too much aspirin it could harm the dog.
Also dogs have trouble digesting coated aspirins so it's best to crush the aspirins up into pieces and place the crushed up aspirins into the dogs food.
The dog will take the aspirin better that way as well.