what type of armature i should use in 12v dc magnetic 2 pole motor to get more than 8000Rpm
Easiest way to characterise the motor is probably to wind the armature with a given number of turns (say, 50 or 100 turns), and measure the speed. That will give you a motor constant (Kv in RPM/volt) for that winding. Now Kv * number of turns will be a constant, so if you need to increase Kv to meet your speed requirements you can do that by proportionally reducing the number of turns.

For example if you measure 3000rpm at 12V with 100 turns, 

Kv = 3000/12 = 250rpm/Volt.
Kv * turns = 25000

To reach 8000rpm at 12V,
Kv = 8000/12 = 667rpm/V
Turns = 25000/Kv = 25000/667 = 37 turns. 

As there are fewer turns there will be room to use a thicker wire gauge, which could be important as the motor will draw more current.

Note that a 2-pole DC motor is probably not self-starting.