Hi may I know what number of baiyun airport shuttle bus must I take to reach the ximenkou mrt stn?

No bus directly leads to the station, but line 5 can be good choice. By that, you need to get off at Haizhu Plaza. Then Ximenkou station is around 2km away. You can take a taxi to reach at a cost of CNY10.

Alternatively, you can go by subway. Take line 3 from the airport to Tiyu Xilu. Then transfer to line 1 to Ximenkou Station.