You can stack them in 3's but you are pushing it with 4+
Better & Bigger ones 
http://www.customthermoelectric.com/
http://www.customthermoelectric.com/powergen.html
These devices utilize the Seebeck effect (the opposite of the Peltier effect) to generate electrical energy. The only requirement is that each face of the device must be exposed to different temperatures; a "hot" side and a "cold" side. The greater the temperature difference, the more power generated. While all of our thermoelectric devices will generate some power, the following devices are the most efficient and capable of operating at 320°C (608 °F) on the hot side. Power generation efficiency can reach 6% with 300°C (572 °F) hot side and 25C (77 °F) or cooler cold side.
The 56x56 mm TEG 
2411G-7L31-15CX1 - $52.00
Watts 21.6 - 3.0 Amps - 7.2 Volts
320°C (608 °F )on the hot side
You use a DC DC Step Up Regulator converter to make 12V 
You wire them in layer series strings groups to get upto A Max of 150V (20 in a string is 144V @3Amps)
Then use 3x 60A MPPT Controllers to Convert the 144V Voltage into 12V With Extra amps (Each time it goes over 12V you gain more 12V amps)
with 3 Stings Layers 
320°C on the hot side
- HOT Side layer
- Mid layer
- COLD  layer
25°C cold side 
the combined total 144V @ 9 Amps from the 3 strings in parallel 
144V fits into 12 12x Times 
3 Amps becomes 36 Amps per string to the  3x MPPTs 
9 amps (combined would be too much for a single 60A MPPT = 108A)
For the aria you would need to make a 5 Second Water to Steam 320°C (608 °F ) Coiled Pipe Heat exchanger for the fire  and Capture every BTU  from the flame to rapidly flash boil water  Were the Volume of water is less than the Contact surface yet there more surface for it to travel across
good example Coiled Pipe Heat exchanger
Cyclone Waste Heat Engine
https://www.youtube.com/watch?v=Ksg-J7IhxUM
https://www.youtube.com/watch?v=6NPpelLCIkk
It there Cyclone boilers / Coiled Pipe Heat exchanger you want to pay attention too
and feed this Steam 320°C (608 °F ) in to a long Square pipe so you can have Seebeck  coving every part of it in 3D 
You have the 4 side to attach stacks too Top Bottom left and right  and slope it at 3° so water collects and ready to go back to the coil for boiling under gravity You could pass this water thow a few Car Matix fan heaters to lose some heat (But put this heat to other uses Heat the home + Dry Wood + Keep a green house warm 
You can also recover the heat form the otherside of the Seebeck   and use to give you a contant supply of hot water 
Surplus hot water can go under the floor of the green house to store heat in thermal mass and cool the return water ready to go though the water cooling blocks on the Seebecks