According to http://www.allicin.com/ , there is about 3.6 milligrams in each garlic clove.
Although this amount may look extremely small, only a fraction of this allicin is going to reach the bloodstream!
Which is extremely disappointing because allicin has got so many health benefits and potentials for getting rid of dangerous diseases.
Nevertheless, if you eat garlic repeatedly every day, you are going to get at the end of the day a suitable amount of allicin.
That is why I recommend eating more at least three raw garlic cloves a day, it is because allicin is going to be present in raw garlic in a much more abundant amounts than in cooked garlic, in fact, you are unlikely to find any allicin at all in a cooked garlic.