I will stay Shanghai for a week visiting international fair show every for 5 days.

According to your situation, you are suggested to purchase Three-day Travel Pass. You can buy another one on the 4th day.
It's sold CNY45 and could be purchased at any ticket booths set at these metro stations. With this pass, you could take all the subway lines without limit in 72 hours after the first use. This pass is not required to be returned after use so you could keep it as a souvenir.