You say you want to run 6 devices that are each rated at 16.4 V and 6.4 A from a 12 V 30 A power supply.
Peltiers look largely resistive to the driving circuit. If one draws 6.4 A at 16.4 V, then just linearly scaling that to 12 V is a good start. That means assume 4.7 A at 12 V. You want to run 6 of these, the total current will be 28.2 A. That's very close to your 30 A limit, so not a lot of slop or room for the numbers above to be off a little. The voltage of a Peltier does have some temperature dependence.
So the answer is it will probably work, but not with a comfortable margin. There are two things you should do:
- Measure one of the Peltiers at 12 V. That gives you the current per device directly. If that is still less than 30 A / 6 = 5 A, then it could work.
- Get a 12 V supply with current limiting, as apposed to one with foldback or a fuse. If the Peltiers try to draw a little more current than you expected, the voltage will just go down a little and no harm will be done. Of course the cooling power will go down too.
For more information on controlling Peltier coolers in general, see How to drive a Peltier element?.