In PRTG "sensors" are the basic monitoring elements and one sensor usually monitors one measured value in your network. On average, you need about 5-10 sensors per server or one sensor per switch port.
As you progress through this short questionnaire, you will go from general to more specialized categories. This also means that once you entered a certain device in a category, you don't have to enter it a second time. If you, for example, enter a firewall in the "infrastructure" section, you wouldn't have to re-enter it in the "applications" section.
