Hard to tell the exact design of the PSU but would suggest a quick mulitmeter check, if the multiple 12V +ves and -ves are connected to each other you can load them as one, if they are electrically seprate +ve1 not seperate to +ve2). Would have expected at least one -ve 12V rail. The actually loading is a bit suck it and see. Basically you want enough current flow to keep the regulator happy without setting anything on fire or running up a power bill needlessly. Starting point would be a standard 0.25W resistor which would be 0.25W/12v = ~0.0208A = 20mA current and 12V/0.0208A = 576ohms. Actual resistor values would be a choice of 560 and 620, with 620 being a bit safer.
If that works and all voltages stay stable you could try running out to 1K for cooler operation, if it doesn't then you need to get something hulkier and look at heat sinks. Also note that some power supplies run a sense circuit that will need to be tied back to ensure stable drive levels.