What is the maximum voltage I can set up on the AFG2125 function generator?
I just want to clarify something for your understanding. When you use 50 Ohm termination, the function generator is pretty much literally putting a 50 Ohm resistor in series with the signal inside the generator. So the max open circuit voltage will be 20V regardless of whether you use 50 Ohm termination or not.
If your load impedance is much higher than 50 Ohms, the termination will have little effect. For example, if you set the generator to 10Vpp with 50 Ohm termination, then look at the signal with an oscilloscope with high input impedance, it will still be 20Vpp. If you set the scope to 50 Ohm input impedance the signal will drop down to 10Vpp.
Hopefully, knowing this, you will be better able to make the decision about how you want to set up the function generator.