Ok so you have a table named Course. you should also have some kind of table called registration that should hold a FK to the Course table.
So let's assume this setup:
tblCourse:
CourseID
CourseName
NumOfSeats
tblRegistration
PersonID (because all your person info should be in the tblPerson)
CourseID (FK to the course table above)
You might be able to do this in one query, but my brain's fried today so I can only think of how to do it with two:
Query1:
SELECT COUNT(*) As taken from tblRegistratiion where courseID = 1
will give you the total of people who have registered for course #1
Then you can take the result from that query and do something like:
SELECT (NumOfSeats - Query1.taken) as remaining from tblCourse
Again with more information from you about your table structure, a more accurate solution could be found, but this should give you the idea.
HTH
Leslie