Looking at visiting Amsterdam August 13-16 2025. First time visit. Have read that Sail 2025 will begin August 20th and they are expecting 3 million visitors. I am concerned that the setup of this event, along with people arriving for Sail 2025, will make it difficult to move around the city and more than usual crowded restaurants and museums. Would anyone know if roads and areas will be shut down ahead of Sail? Or if the city is expecting large crowds before the event?

There will be a lot of people, but many will be day visitors only. I can't see your dates being affected by it. It is confined to the dock areas behind Centraal Station, but the station remains open.
A shame you won't be there to enjoy it though, it is good fun, you can watch the sail-in as the tall ships sail up the canal to Amsterdam, and can visit the ships, big and small once they are in harbour, or take a boat trip around them. Lots of onshore entertainment too.
Some of the ships from my home city of Harlingen will be there, including the Witte Swaen a replica of Barentsz ship.

Thank you! That helps me understand where the festival is staged and how it is managed.

We were in Amsterdam in August 2015 and extended our stay by a few days to see the Sail festival. We spent a whole day on the docks near Centraal, walking around and going on several ships, it was great fun. I loved the tall ships, they were magnificent!