Everyone asks how many devices 100 free sensors cover. Wrong question. Sensors are spent per value, not per device - here is the real maths, with real device examples.
The most expensive misunderstanding in PRTG is one sentence: "one sensor equals one device." It does not. Paessler defines a sensor as one measured value - the traffic of one switch port, the CPU load of one server, the free space on one disk drive. Once you plan with that definition, the free 100 sensors stop looking small and start looking like a budget you can actually manage.
A device in PRTG is just a folder. Sensors are the live checks inside it, and the licence counts sensors. One sensor watches exactly one thing with one method: a Ping sensor, an SNMP Traffic sensor on port 3, a WMI Memory sensor. Paessler's own published average is about 5 to 10 sensors per device, or one sensor per switch port - use that average for planning and you will never be surprised.
Three habits burn budgets. Auto discovery generosity: it creates every sensor it can - pause and delete before you fall in love. Per-port thinking: graphing every switch port "just in case" converts one cheap switch into your biggest sensor consumer. Short intervals: intervals do not cost extra sensors, but they cost probe load - a different budget people confuse with this one. Review the sensor list monthly; anything nobody has looked at in a month is a candidate for deletion.
Write the list on paper first: servers fully, network gear lightly, everything else barely. A workable small-office budget: 3 servers at 8 sensors (24), firewall and core switch (8), two access switches uplinks only (4), NAS with disks (6), a handful of printers (8), website and mail checks (4) - around 54 sensors, leaving headroom for the day something new matters. That headroom is the real prize: a budget with room is a monitoring system you can grow; a budget at 100 of 100 is a wall you hit during an emergency.
Every sensor in this guide is created live in the video - installation, discovery, CPU and RAM, bandwidth and firewall status.
Watch Full VideoYes - a sensor can carry several channels (for example traffic in, traffic out and errors on one SNMP Traffic sensor) and still counts as one sensor. It is the sensor, not the channel, that is licensed.
You simply cannot add more until you delete some or upgrade the licence. Existing sensors keep working. That is why planning the list before auto discovery spends it for you matters.
Sensors that exist count, even if you rarely look at them. Deleting sensors you will never act on is the healthy habit - export or note the configuration first if you think you may rebuild it later.