In VBA, you could do a recordcount in the AfterUpdate of the form to see how many records are attached to that parent record and if 30, then change the form property "AllowAdditions" to false.

On the other hand, you could use a query to filter the # of records a report shows to 30 as well so the subform could take all relevant records that go to the master form and then only print 30 in the report before having to go to a new page/report.

Correction, to do in the form, would be in the After Insert event.

And, if that were the route you choose to take, you'd also need to check the record count every time the form opens and every time the parent form moves to a new record.

I dont want it to go to a new report. I want it stop at 30, and no more. How do I do the after insert event with VBA?

In that event directly would be something like:

Details

Private Sub Form_AfterInsert()

    If Me.Recordset.RecordCount > 29 Then
        Me.AllowAdditions = False
    End If
    
End Sub
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That would lock the form so no more records could be added. But again, you'd need to check when the parent form loads or changes records to determine whether the subform should be able to accept new records. So, I would turn that code into a separate public function that could be called from anywhere:

Public Function LockIfFull()

    If Me.Recordset.RecordCount > 29 Then
        Me.AllowAdditions = False
    else
        Me.AllowAdditions = true
    End If
    
End Sub

Then in the property sheet of the subform, I would just call that function anywhere the number of records needs checked by typing: =LockIfFull() into each property in the sub form's events tab - see image below:

If you allow users to delete records in the sub form, I would disable deletions in the form itself and put a button somewhere so you can control it with code. For example:

Details

Private Sub DeleteBtn_Click()

     ' Make sure they actually want to delete the record
     If MsgBox ("Are you sure you want to delete the selected record?", _
          vbYesNo + vbQuestion, "Confirm Deletion" ) = vbYes Then

          ' If Action/Delete Warnings are enabled in the database, turn them off since we know it will fire
          DoCmd.SetWarnings = False
          
          ' Use SQL to delete the selected record
          CurrentDb.Execute("DELETE FROM tablename WHERE recordid=" & recordid)

          ' Requery the table to get an updated record count and call the function to enable or disable additions
          Me.Requery
          LockIfFull

          ' Turn Warnings back on to be safe
          DoCmd.SetWarnings = True
     End If

Exit Sub
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* Replace "tablename" and "recordid" as approriate

This would ensure that when the main form loads, it loads this form and it checks if it's full. each time you go to a new record in this form, it verifies if it's full. If you delete a record, it verifies if it's full. If the main form moves to a new record, then this will start over and verify if it's full. This way, if multiple people are working in the same form ever, it will frequently verify when it gets full.

The AfterInsert event would go in the subform that I want to control?

Yes, that would stop them from entering any more rows at that time.

Donald How do I get the Public Function in? I can't remember. Is it a separate VBA module?

As long as you're only calling it from within that sub form, you can put it in that forms code. If you call it from the form's property sheet like the image I showed, you could just replace the Private Sub Form_AfterInsert() with the Public Function...

Oooh, I just noticed a typo in what I had above for the Public Function, the last line should be "End Function" and not "End Sub"

I got it to work as I needed it to.

Thank you