At the beginning, you are trying to remove the creep from a string, and the number of shots that takes will depend on the string. A custom pre-stretched string may be fully settled after a dozen shots, while a cheaper machine maded model might need hundreds of shots. The string material will affect the amount of stretch. Material like Ultracam has very little stretch, so once the creep is taken out, it remains very stable in length. On the other hand, a material like D75-thin, is a more stretchy string, and will tend to move a bit more, even if pre-stretched. It is also more affected by heat and humidity.