10BASE-T and 100BASE-T require 2 wires to transmit and 2 wires to receive, 4 wires total. Actually it is better to call them 2 pairs since it's a balanced arrangement in which each pair signals come and go in reverse phase from the interface transformers and wires of each pair are twisted together so that any picked up noise can be canceled out.
1000BASE-T (Gigabit Ethernet) uses all four pairs, that is, all eight wires of a CATx cable.
And, as a earlier poster explained, there is 10GBASE-T1,
which is 10G over a single pair! Wow! Who knew!?
"The 88Q4364 is manufactured using a standard digital CMOS
process and contains all the active circuitry required to
implement the physical layer functions to transmit and receive
data on a single balanced twisted pair."
