i'll arrive at pudong airport at 23.30. is the customer service open to buy public transport card?

No, it will be closed, for the last subway train leaves the airport at 22:00. After that, the customer service will be stopped.

So may I buy this card the day after at east nanjing rd or people's square?
Tha card will cost 20 plus the recharge, won't it? How much do you suggest to recharge it for 3 days in Shanghai?

Yes, you can buy the card at these two subway stations during their service hours.

Yes, the card has CNY20 deposit and recharge part. When you do not use the card, you can get the balance and the deposit fee. For your three days stay, i suggest you top up the card with CNY100 around.

Is the card personal? We're 2, should We Buy 2 cards or one is it enought using it twice? Thank You so much!!

Yes, the card is personal. You two cnanot share one card to take the bus or subway. Therefore, it is necessary for you two to purchase two cards.