No, a 9 lb turkey would feed about 4 people and, despite a previous answer, they would not have to be gluttons. Most recommend, for larger birds, a ratio of about 1.5 lbs per person so using 21 people you are looking at a bird weighing in at approximately 32 lbs. Although these can be found many people I know would try and purchase 2 birds of approximately 16 lbs each (err on the high side) if they have the ability to cook them at the same time. I would choose the size based on your ability to cook them, if you only have an oven then you need to look for the larger bird, if you have a double oven or something like a table top oven then 2. Also, if you are planning on deep frying them I would go with two 16 lb birds as the cooking time is significantly reduced and the size makes them easier to handle.
This is, of course, assuming that you are having a regular meal with turkey as a main course. If there are other meats involved in the meal then you could get by with a smaller bird overall however I would not go lower than a 20 lb turkey for the number of people you are dealing with.