It’s best to purchase a humidifier that has a built-in hygrometer (the instrument that measures ambient humidity) and can be set to run at a specific humidity. The reason for this is that molds start growing in the home environment at 60 percent humidity, so ideally you would keep the humidity as high as possible for your bird without exceeding the level at which molds will start growing.
Purchasing a humidifier that will allow you to set it at 55 percent humidity will ensure that your bird lives in as humid an environment as possible without mold growing. Birds living or staying in outdoor aviaries can live with higher humidity, of course.