Should I mention the number of followers of my blog in my SOP for grad school?
(a) There is not harm in mentioning it. Of course, how they interpret it is up to the department. In my cohort (social sciences), there was one student with a blog that he runs that generates quite some views. I am sure he included that (views) in his application.

(b) I would imagine it can only have a positive effect on your application.