I also think you're in for a surprise when you price out 8 pots rated for >1W. They'll need to have low temperature drift, since they'll heat up under load, as will your 200Ω resistors. If you need precision and accuracy, you might need individual current or voltage controllers on each leg.

One of the nasty things about rheostats or pots used as rheostats is that their rated power only applies for the entire resistive element.

In the graphic, R1 and R3 are 6 ohm 6 watt rheostats, and R2 and R4 are 6 ohm resistors.
Note that decreasing the resistance of R3 to 3 ohms in order to cause R4's dissipation to increase causes R3's dissipation to change to 5.3 watts, but that's across half of the element, which is rated for only 3 watts.

 

One has to wonder if people posting queries on this site have ever looked at the site before! Time and again, it is pointed out that you only get a sensible, useful answer if you pose a sensible, useful question!

Telling the forum a small part of what you are seeking, and expecting a reasonable reply is like leaving three wheels off your car and expecting it to drive down the road.

If you want a good, practical answer to your problem, then you need to state EXACTLY what you are trying to do, in clear terms, so that people with more knowledge than you can give you the benefit of their expertise, otherwise they are just poking around in a morass of guesswork.

 

Peanut gallery here:

Forgive me if I'm wrong, but doesn't changing the number of parallel resistors change the current? I mean, assume all these resistors (for sake of argument) are 1K ohm resistors.

[edit] (OK, NOW I got my numbers straight) Eight of them in parallel present 125Ω overall. At 12 volts, 125Ω, that's going to be 96 mA at 1.2 watts. Meaning each resistor is going to have to dissipate 144 mW. If you suddenly drop half of the resistors you're going to change the resistance to 250Ω at 48 mA and 576 mW overall. With everything changing every time you switch a load in or out - how do you maintain a fixed value of anything?

Like all things - do the math. And make sure you've done it right! Like I didn't (if you saw my post before I edited it to reflect the correct numbers. NOW I'm confident I have the right numbers. And this is basic stuff too)

 

Does that help? Each resistor is essentially a heater for an 8 up test. I want each resistor to dissipate 500mW, and hopefully not have to re-calibrate every time I take a unit on or off.

Petkan:
You can buy for less than $3 an isolated, regulated 12V/2A power adapter. Once the voltage is regulated, the power dissipated by an active resistor will be stable (V^2/R). Essentially you need an "ideal" voltage source i.e. one that maintains its voltage vs change of current.