A formal gown can be taken in 2 sizes pretty easily but taking in more than 3 sizes requires costly recutting and potential redesign to make it work. Making a drastic size reduction makes it difficult, if not impossible, to keep specific details of the gown.
If your dress needs to be taken in several sizes, be sure to ask about the sewer's experience with such alterations. You want to be sure an inexperienced sewer doesn't ruin the dress.