At what current and voltage should I run my TEC/Peltier modules for max cooling effect?
If you put the modules in series the current will drop.

Read the datasheet for your device, here's one for the TEC1-12706 

If you put 8 in series each one will have approximately 1.5V and it's likely you will draw less than 500 mA current. Notice that operation at this level does not even appear in the graphs. 

If you connected 2 in series then you'd have approximately 6 V per device, and you would have something like 2-3 A flowing, so could connect at least three pairs to your power supply, but might not be able to connect 4 pairs without exceeding your supply current rating. 

You are giving up a lot of cold potential in this configuration since each device is limited to about 18 W and you could only use 3 series connected pairs on your power supply. This gives about 54 W total.  

For 8 devices operating at 12 V you'd expect to be able to get somewhere near 72 W per device for a total of about 576 W from the devices. 



Your current power supply is not rated to run 2 of these modules in parallel. To test out your heatsinking I'd suggest you connect only 1 device, and sort out you hot side heat dissipation, then replicated that 8 times larger.