although I have yet to see anything solid to dictate how much is too much, this is what most say:
from here
Carine: Garlic for Dogs?
According to the Whole Dog Journal, small amounts of garlic not only act as a natural flea repellant, but garlic can be used for its wonderful antifungal and antibacterial properties. It also promotes the production of white blood cells thereby acting as an immune booster for dogs with low or compromised immunity and may benefit dogs with diabetes by helping reduce blood-sugar levels.
What makes garlic so great for dog health problems? Allicin appears to be the active component in the root bulb (cloves) of the garlic plant which trigger its healing properties. Allicin is formed when alliin, a sulfur-containing amino acid, comes into contact with the enzyme alliinase when raw garlic is chopped, crushed, or chewed. Heating garlic will lessen the medicinal capabilities, but naturally dehydrating it won’t. That is to say the garlic used in a nutritional supplement, or garlic found in one of our pet food mixes is simply raw garlic that has been crushed and dehydrated.
Despite its healing qualities, Garlic contains a compound named thiosulphate. In extremely high levels thiosulphate can be a dangerous toxin that cause hemolytic anemia in dogs. But we’re not talking about garlic dog treats, supplemental garlic, or healthy table scraps that may have included fresh garlic in the recipe. We’re talking about situations where your pet sniffs out several bulbs of garlic you were about to use for a giant batch of homemade spaghetti sauce for the whole neighborhood and winds up eating 50 cloves in one sitting. We repeat . . . it would take up to 50 cloves for garlic to be harmful to your dog! 50 cloves of garlic wouldn’t be a good idea for anyone, let alone your dog.