why not use 6 phase or 12 phase in transmission line?

For 3 Phase system the phase angel between transmission line is 360/3=120 Degree. As the number of phase increases the phase difference will decrease and more number of times transposition is required. It affects the Installation cost of transmission of Transmission towers.
And 3 is the minimum phase by which we can make multiple by requirement.

The electrical efficiency gain of using more phases is not enough to compensate for the additional cost of wires and towers to support them.

An interesting project would be to calculate how much copper wire (e.g. kg /kilometer} is needed to carry a certain amount of wattage for various phases - e.g. 2-phase, 3-phase, etc.

Jay Kosta
Endwell NY USA

Hafeez ur RehmanFor 3 Phase system the phase angel between transmission line is 360/3=120 Degree. As the number of phase increases the phase difference will decrease and more number of times transposition is required. It affects the Installation cost of transmission of Transmission towers.
And 3 is the minimum phase by which we can make multiple by requirement.

yes. i agree but by using 12 or 6 phase maximum power is transmit without any boosting transformer

A transformer has nothing to do with it.
With 3-phase, the resulting voltage is obtained by using ANY 2 of those wires because EACH wire is 120 degrees out of phase with the other 2 wires.
If more than 3 phases are used, then the voltage across different wires would depend on which 2 wires were used. And that makes it VERY DIFFICULT (and expensive) to install the wiring needed to have the proper voltage and to 'balance the load' so that each wire is carrying the same power load.

Jay Kosta
Endwell NY USA