In PRTG, sensors are the basic monitoring elements. One sensor usually monitors one measured value in your infrastructure, for example, the traffic of a switch port of the free space of a disk drive.
On average, you need about 5-10 sensors per server or one sensor per switch port.
All licenses of PRTG only differ by the maximum number of sensors that you can use for monitoring.
Note: When you buy one license now, you can upgrade to a larger one at any time by paying the price difference.
About devices and sensors: