Number of Funded PhD Positions available to applicants
At US universities, it typically doesn't work that way.

The department usually doesn't think of having specific PhD positions that have to be filled.  Rather, admissions is done on a department-wide basis. Each year, the department sets a target for how many new students they want to admit, based on available resources.  Let's say for example that this number is 20.  

Now, most applicants are applying to many different programs, so not every accepted applicant will actually come to study in this department.  Thus, if they want about 20 new students, they will offer admission (with or without funding) to some larger number, perhaps 40 or 45 (assuming there are at least that many qualified applicants).  This number would be chosen based on past experience as to what fraction of accepted students choose to attend, and can be varied if they have applicants whose applications suggest that they would be particularly likely or unlikely to attend.  A few additional applicants might be placed on a waiting list.

But it's entirely possible that the number of students who actually choose to attend is larger or smaller than 20.  If it's smaller, they could try to admit some more people from the waiting list, but that still may not bring them up to 20.  That's okay; they will just have a smaller cohort that year, and probably try to use a larger target number next year.  If more than 20 accept the offer of admission, then the department just has to come up with the funds to support them, and use a smaller target next year.

Also, seats aren't necessarily restricted by research area.  In mathematics departments, for instance, departments won't typically break down their targets as "we want 5 students in algebra, and 3 in number theory, ...".  They'll just admit 40 good students and let them sort out their areas of study after they arrive.  

So the number you are looking for may not exist.