AFAIK, JTAG is requires 5 pins (Wikipedia article):
TDI (Test Data In)
TDO (Test Data Out)
TCK (Test Clock)
TMS (Test Mode Select)
TRST (Test Reset) optional.
Let's add two more Power supply Pins (Vcc and GND).
If it is of 7 pins then why most JTAGs connectors come with 10 pin or 14 pin or 20 pin variants.
Most of the pins are either NC or GND. Why so many GND pins are provided? Is there any special reason?