I believe we can't. We can only see all posts regardless of which category.

AFAIK, there is no easy way, but you could press that view more button in Special:UserProfileActivity until you make it to the bottom,

and then type the category name in that “find” box (I don’t know what its called xD) and it should give you the number of all the posts under the category, but obvious, its not easy to do if the user has many posts.