Basically, our per-device licensing model means that you pay for the number of devices you want to monitor.
Getting into technicalities: a "device" is a unique representation of a host on a map. The device may represent a single service (HTTP, DNS, FTP, etc.) on a host, or it may represent a group of services all running on the same host and at the same IP address. If you create a device for HTTP, a device for DNS, and a device for FTP, that will count as three devices, even if they are served from the same host.
On the other hand, if you create a device that includes HTTP, DNS, and FTP running on the same host, it will count as a single device. Furthermore, if you have two or more SNMP or Ping probes whose parameters are exactly the same targeting a single host, they will count as only one device, no matter how many identical instances there are.
Devices with a probe types of "Non-polling" are excluded from Intermapper's count.