If you join two tables and use count, you'll have the count of the result set, not the table. In your example you would get the total amount of students whose address have id 20.
So if you want to get the total count separately, you can use for example scalar query for that, like:
SELECT main.*,
arc.*,
(SELECT COUNT(*) FROM student_main) AS TotalStudentAmount
FROM student_main main,
address arc
WHERE arc.id = 20
AND main.stu_no = arc.stu_no
Another thing, don't use nolock unless you
really have to (for good reasons).