You may want to consider how many sensors are appropriate for your space. Consider this article to help determine the best amount for your needs.
PurpleAir sensors only measure the air pulled into the sensor (roughly a 2-inch by 2-inch area). This means you should think of data from a sensor as representing the particulate matter (PM) at a particular point, not an entire area. The more points (or sensors) in an area, the better spatial resolution of PM will be in that area. Since air quality is a complex issue and unique to each location, it can be difficult to determine how many sensors an area needs or where to place them. To explore this on your own, you can search for areas of comparable size and desired coverage on the PurpleAir map, and get an idea of how many sensors you may need. Other things to consider are localized sources of air pollution, weather patterns (i.e. predominant wind directions and wind speed), and local points of interest such as hospitals, libraries, schools, parks, etc.