Yeast washing and rebrewing

So Ive washed my yeast down and I have 1 cup of liquid yeast in the fridge. Id like to begin seperating this 1 cup out to an adequate amount for a starter to ferment 5 gallon batches. So... the real question is how many billion yeast cells can I expect in 1 cup of yeast. Anyone have any experience doing the same? I plan to brew a 20 gallon batch in 4 separate fermenters and would like to have my yeast ready.

Thanks

While I'm no expert on the subject, I would have saved a bit more.

http://www.mrmalty.com/calc/calc.html

Mr malty linked above gives 330 ml, so around 1 1/2 cups for 20 gallons at 1.048 gravity.