A simple equation is X*Y*Z = Yeast Cells Needed.
X= pitch rate in million cell per ml (for ale about 0.75 M/ml, double that for lagers)
Y=total wort volume in ml (5 gallons is about 20,000ml)
Z=degrees Plato (roughly the last digits of the OG/4, so 1.060 = 15°)
Plug it all in...
0.75*20000*15=225,000 or 225 billion cells
Now, we need to know how many cells/ml are in our starter.
Luckily, there plenty of research and information out there. This article is my favorite.
It's got a stir plate prepared starter having between 180M and 360M cells/ml. That means you'll need about 1L of starter.