1- You can't have 2 identity key on a table.
2- If you have 1 identity column and you want to insert 1000 records you can do like following.
DECLARE @Counter int
SET @Counter=1
WHILE (@Counter) <= 1000
BEGIN
-- Wrtie your insert statement here
SET @Counter = @Counter +1
end
3- If you have a table having 1 column of type identity in that case you can add a row like following.
INSERT INTO TABLE_NAME DEFAULT VALUES