More than two million people a year visit Potsdam but, luckily, there’s enough to see that it doesn’t feel too crowded. The whole area is about 500 hectares large and it’s possible to walk through the parks and the gardens and see the exteriors of the buildings for free.
You need to buy a ticket to go inside most of the palaces. Because Sanssouci is so popular, you’ll be allocated a timeslot for a tour. It’s better to get to the site early if you are short of time, otherwise you may miss out.
Aside from Sanssouci and New Palace, the highlights are the Orangery Palace, Charlottenhof Palace, and the New Chambers. It can easily take all day to see them properly and also stroll through the gardens. As Potsdam is just a short train trip from Berlin, my recommendation would be to allow the whole day to explore.