Need a message to pop up on a form but based on a count from a table

Hi!

I have a access database that has a Table called "tblClassRegistration".
The Class Registration table captures data entered from a FORM called "frmClassReg". Each class has a particular code, or ID that is unique to that class. For instance, BLTI-MON, is the code for Ballet I class on Monday's. The codes, such as "BLTI-MON" come from another table called "tblCodes". This Codes table contains the Codes and a number that represents the maximum amount of students (LIMITS) that can be enrolled in that class code. For instance, the BLTI-MON code can only have 10 class members.

What I need to happen is on the "frmClassReg", I need a message to appear in red that says this Class is full when the class has reached its limit.

I have an idea how to do this, but what complicates matters is that I have to also filter it using the Session Start Date. Session Start Date is entered on the form and transfers over to the "tblClassRegistration". For instance, I may have 20 students signed up for BLTI-MON, but 10 of those may be for Session 1 and the other 10 for session 2. That's where filtering by StartDate comes in to effect.

Any help, tips, or advice would be greatly appreciated.

Thanks!