How many impressions do you have to have at the dentist for getting dentures?

1-2. Most of the time, only one is needed per arch. You will probably have at least a couple more appointments before getting the final denture.
Usually 2. Usually just 2 for each jaw. The first is a priliminary one , usually made in alginate which sets in about 2 minutes. The second , or final, impression is made using a customized tray made from the first impression so that it fits just right for your mouth. This gives us the best impression of your existing tissue / bone contours and muscle attachment areas. These are critical for an optimal fit.