How to calculate output of TEG
Knowing what the delta T is challenging. The temperature of the peltier, and heatflow across it needs to be known. Usually the most challenging part is calculating how much heat needs to be moved, because that usually involves a larger thermal system with insulation (or just air with an object) and it's hard to know exactly how much heat needs to be moved.

If I want a 20C deltaT with one module, I draw a line on the graph from 20C (this is on the 27C graph, meaning that the module is operating at 27C). 

I then see that if I provide 1.2A, I get 5W of cooling, 2.4A give me 10W and 4.8A gives me almost 20W. 

To get 1.2A, on the input voltage graph, I'll need 2V. 
2.4A I'll need 4V
4.8V I'll need 6V.

If I have two modules in series, it halves the deltaT so I'd need only 10C on both to get a 20C drop. The current is the same through both, but you need twice the voltage because you have two of them to get the same current. Here is a comparison:

One module at 20C deltaT:
1.2A = 5W of cooling (2V)

Two modules 20C delta T (10C each):
1.2A = 6W of cooling (2V x 2 = 4V)



The other problem that I am not accounting for is self heating. Peltiers generate heat (they have resistance) and this heat moves to the hot side. If you do put modules in series, the heat from one will need to be pumped out by the other. And this is where it gets complicated. I will say this, it's better to have more peltier on the bottom of the stack (so three with two on the bottom and one on the top) to move the heat generated by the first. This is a common arrangement and I've used it several times. 

I hope that is enough to get you started. You may also want to use the 50C graphs in your calculations because the peliters are likely to be hotter than 27C, but that depends on the setup.