Hi Rifakat, Thanks for helping me ;-) I successfully implemented the constrain, and now the user cannot save the submitted parent form if the child items accedes the limit. User should delete the extra child items manually. As you know the @api.constrain function called on form save, so do you know if I can even prevent adding more items before reaching the form saving? (I am able to show the warning after he adds but not able to delete the added child rows)

you right. constrain will be checked when saving the record while user has already added all items. you can also create @api.onchange('order_line'), to raise a warning, but as you may know, onchange would only raise warning and will not restrict user to add more items. In your case you will need @api.constrain() as well as @api.onchange(), constrain to restrict saving records, and onchange to warn user that he is doing wrong. Thanks.