We are going to stay in Guangzhou Bauhinia hotel, Which bus should we take from the airport?

You should hurry up upon arrival to catch the shuttle bus Line 5. The last bus of this line will leave at 01:00. You could take it and get off Haizhu Square, and then walk up along the Jiefang South Road for about 400m to get to the hotel.
If you miss the last bus, you'd better hire a taxi driving to the hotel directly.