How many 12 V 100 Ah batteries do I need for overnight?
So, with your clarifications, this becomes easy:

  480 Wh Fridge
+ 308 Wh PC
+  80 Wh = 50 W · 16 h light bulb (christ! get LED lights)
+~ 50 Wh "Stuff" for 16 h
========
  918 Wh


So, you need 918 Wh of energy on your net.

A 12 V, 100 Ah battery has a nominal capacity of 12 V · 100 Ah = 1200 Wh. But you can't assume that's true – batteries age, and are load-dependent. So, we'll easily have to "reduce" that number to ca 800 Wh for "reliability" reasons.

Then, you have non-perfect voltage conversion efficiency. Let's say, 85%, that'd be pretty good for 12V –> 120V or 240V, whatever your devices use. 
Incorporating that efficiency, your primary energy usage would be ca 1080 Wh.

That's more than 1 battery, and less than 2. I'd argue with 3 you'd be on the safe side.