Given that you are probaby referring to the best odds guaranteed promotions that many online bookmakers operate, then you need only use one bookmaker to ensure you get the promotion. Of course, if you are really shrewd you will be able to scan every single bookmaker for the best odds before placing your bet thus enhancing the chance you get the best odds every time. This isn't really a viable exercise given the time it would take, which is why using one of the best bookmakers for best odds guaranteed is your best bet.