limiting amount of records

Hi everyone,
I'm new here and I need some help.
I have been developing a registration access database and got stuck at one spot.


I have:

• classes, instructors, and students tables
• class rosters, instructors-classes, and students-classes query's
• class roster, instructor records, and student records forms

all in my database right now. The student records form is being used to register students to a particular class. The student records form shows student information and has a subform with students schedule. We register class based on class item # (a 4 digit class ID)

Here's where i'm stuck at...
I need a limit on a number of records that can be applied to a particular class (item #). For example class 4000 can have 30 students, so I need set a limit so that when we do registration we don't go overboard.

That's number 1. The second issue is that we have combined classes. For example: classes 4001 and 4002 TOGETHER need to have a combined limit of 30.

Is there anyway i can set a limit through a vba code? like a

Private Sub ITEM___BeforeUpdate(Cancel As Integer) or anything at all?
any help will be greatly appreciated...