Electronic – 6 x TEC1-12706 in series cannot reach desired amperage, why

Thermo-Electric Cooler efficiency "is a function of ambient temperature, hot and cold side heat exchanger (heat sink) performance, thermal load, Peltier module (thermopile) geometry, and Peltier electrical parameters." And "A single-stage TEC will typically produce a maximum temperature difference of 70°C between its hot and cold sides. The more heat moved using a TEC, the less efficient it becomes, because the TEC needs to dissipate both the heat being moved, as well as the heat it generates itself from its own power consumption."

So a heat sink on the "hot side" alone isn't enough. The cold side gets cold quickly, having little thermal mass, and efficiency falls to a very low value. Also as the hot side warms up, each junction's internal resistance increases, drawing less current.

TECs are most efficient when both sides are the same temperature (and little heat is being moved.) The best overall working efficiency (in terms of pumping heat) may be up to about 13% of Imax. Beyond these currents, resistive heating losses increase greatly, and the device spends more power moving heat it generates itself.

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.