What is the sense area? or how many of these should I buy for a 1125sqare mtrs space

As is true with all sensors and loggers, the data being recorded and shown by the device is only for the location of the sensor.  The further away you move from the location of the sensor increases the changes that the conditions will differ.  We recommend that customers first use a single logger to experiment mounting it in different locations within the space being monitored.  This will help you determine how much variability you have in the space and give you a better idea as to how many devices you need to give an accurate representation of the environmental conditions in the space.