How many days I need to stay in Xi'an if I want to see all top attractions?

If you just want to cover all the places you mention, you can finish them in about two to three days.
One day is spent on the terra-cotta museum and wandering around the city in the late afternoon.
The second day can be used to see the Shaaxi History Museum in the day time and the Wild Goose Pagoda at night.
The third day, you can visit the Bell and Drum Towers and the Muslim Quarter to taste the local snacks. In the afternoon, you can visit the City Wall.