Why JTAG connectors are available in 10pins/14pins/20pins when JTAG is of 5pins

I don't know the exact reasoning for JTAG, but when high speed signals are used and according to the best practices, you should put a GND in between every signal of a flat cable. JTAG can be considered as a High Speed Signal.

Multiple GND wires are used to avoid crosstalk between the signal lines. They avoid capacitive coupling between adjacent lines. And they also provide a separate return path for every signal. Indeed, in high speed signals, the return current "prefers" the path of least impedance. That path is, for high speed signals, the closest GND. Thus the different signals will have different return paths and that avoids crosstalk of the return path.

The final goal is to guarantee good signal integrity, reduced emissions and a better immunity to external disturbances.