There's just no way to know the answer to this question...serious anarchists tend not to fill out surveys, institutions never or rarely ask people about that, and worst of all its unclear now adays who is and isnt an anarchist..
By my standards, an anarchist is either someone who calls themselves an anarchist (and this is a problem cuz by that standard someone could be a nazi and an anarchist...) or someone who lives like an anarchist (?)
If i were to guess, id say less than .001% of people in the world (about 8 million according to google) identify with anarchism in a serious manner. Its a very un-popular ideology or religion. Marxism and far right nationalism (the other two extreme political ideologies) are waaaaay more popular in most countries.
edit: forgive my failure with decimal places...