A build-up is only needed if there is insufficient tooth structure to hold a crown. Generally this is the case if there is significant decay, fracture, or large old fillings. Also, if the tooth has had a root canal. In general about 1/2 to 2/3 of teeth I crown need a build-up. For the most part, crowns being done for cosmetic reasons do not need a build-up because the tooth structure would be mostly intact and sufficient to hold the crown.
In theory, re-crowning 20 teeth can be done in 2 visits about 2-3 weeks apart, if there is a rush to do it. However, this would be an all day event and very exausting for both the doctor and patient. Also, it leaves very little room for error if there are any unforseen issues. When doing this many crowns, you usually want to do a try-in and then send them back to the lab for final modifications and glazing. I feel I can do a much better job when I break it down into smaller sections, otherwise I feel like I'm rushing and very tired by the end of the appointment.
This is a lot of time and money and it is not something you should rush. You want this to last a long time. I prefer to do this the following way: 1)Prep teeth in two appointments (can be consecutive days). 2)Wait a few days to evaluate the temporaries and make any adjustments. 3)Once patient is happy with looks and feel of temps, then take final impressions. 4)Try-in crowns and make adjustments and send back to lab. 5)Deliver crowns.
This would take a total of 5 appointments in about 6 weeks.
Good luck
Dr. T