"Not enough" is a correct answer, but in case you are still looking for a more quantitative measurement, the answer seems to be one - Leon Czolgosz shot President McKinely in 1901. Other presidential assasins were motivated by various other political beliefs.
When the view is expanded to well known would-be assasins, we might arguably include Guiseppe Zangara, based on his alleged* statements while on trial, "I have the gun in my hand. I kill kings and presidents first and next all capitalists, " and, "Hoover and Roosevelt — everybody the same."
While Zangara, so far as I can tell never professed anarchism, his analysis seems to fall within the realm of affinity-with.
* This is based on my previous knowledge of US History (moderate, but not scholarly), and a quick study of internet resources about known presidential assasinations. I chose to not include a Symbionese Liberation Army sympathizer, or the suspected assasinations where there is no proof or suspects. All the same, for actual assasinations, we are responsible for fully a quarter of them!.
