This will vary with some models but typically I cover the bottom of one wing and then the top. Then I cover the other bottom and then it's top side. Therefore 4 pieces would be used. If you have multiple dihedral breaks (polyhedral), like on many gliders, there would be more pieces.
An alternative is to do both bottoms and then both tops.
You may also want to do the tips first, depending upon their shape.
Many variables and just need to study/plan a little before jumping in.
There are also times when you do the center section first and then the outer wings.
http://www.rcuniverse.com/forum/fb.asp?m=3746641 and later may help.