The minimum usually is 2.5k. You can probably go up to 10k.
Lets assume you have a 2500ohm pot, you want 10v maximum across the pot.
This results in a current of I=E/R = 10/2500 = .004 or 4ma.
Now you have 2 volts to lose somewhere, so put a resistor in series at the 12v end.
The resistor will have to drop 2v at 4ma, R=E/I 2/.004 = 500ohms.
For any other pot value, use the same formula, the input impedance seen by the pot slider can usually be ignored for this calculation.