Sorry, i forgot to reply. Got a bit caught up. Finally used the following with success!\
IF ((@OBJECT_TYPE = '13') AND (@TRANSACTION_TYPE = 'A'))
BEGIN
DECLARE @IN11ITEM AS NVARCHAR(5)
SET @IN11ITEM = (Select distinct 'true' from INV1 t0 where t0.DocEntry = @LIST_OF_COLS_VAL_TAB_DEL and t0.[LineNum] > 9)
IF @IN11ITEM = 'TRUE'
BEGIN
SET @ERROR = 1
SET @ERROR_MESSAGE = 'You may not have more than 10 lines in a document'
END
END