Hi - I too was a bit stymied by the whole sensor location question - here is a link to a recent post - theres a few useful tidbits in the 3 minutes of reading!
Keep in mind that temperature sensors are rarely single function - often they are 'multi' sensors capable of motion, humidity, illuminance and even others! Since a temperature sensor is not 'real-time' - that is to say it is sending it's status at some pre-defined period or schedule it can also be affected by the location not just by the height but location relative to heaters and vents.
Last, I would really consider the 'how many' issue - initially I wanted one in each room of the house only to actually see over time the variance was really minor! Placement in the hottest and coldest points of the house and of course the temperature provided by your thermostat (commonly located at shoulder height, central to a home) will give you the Delta values above and below your central reading!
In the end, I used an 'averaging' of all my sensors with a display of tenths of a degree. I also learned not to overdo my thinking - a rule or action your Hubitat can do, can be specific to just one room so in the case of one of my sensor locations being extremely odd (the office floor) I just put a single sensor there that controls a nearby portable heater. I tend to overthink!
