Just a quick post since I speak with many network engineers and I notice some confusion about this topic.

Are all 4 pairs of an Cat5e cable used?

The answer is.. it depends.

For gigabit speed 1000T all four pairs are used .

If 100TX speed is enough we can of course split the cable and double the connections without pulling more cables:

In my specific case I’ve used a single cable drop in the garage to connect a humidity/temperature to a 1wire bus sensor while allowing a future connection of an Ethernet device.