Get new ID for inserted ID that comes from another table?
It incremented by 50 because you added "ordID" to @idmax and ordID = 50.

Maybe you wanted an incrementing number using row_number?

DECLARE @idmax INT
SELECT @idmax = MAX(OrderNumber) FROM OrderNumbers

INSERT INTO OrderNumbers
(OrderNumber,
 OrderGUID)

 SELECT
 row_number() over(order by ordID) +@idmax,
 NEWID()

 FROM orders



 SELECT * FROM OrderNumbers