The code seems to work for me. Each record in the main form can have 15 records in the sub form. Where are you using the code.
When I go to a new record on the main form I cannot enter anything into the subform.
To fix that, try following code the main forms After Update event.
Code:
Me.YourSubFormCtrlName.Form.AllowAdditions = Me.YourSubFormCtrlName.Form.RecordsetClone.RecordCount < 15
Replace YourSubFormCtrlName with the name of your sub form control. This may or may not be the same as the name of the form being shown as a sub form.