How many lithium cells are there in a MacBook Pro 15"?
It has 6 cells.  However, they are of varying sizes.



I was able to find a shipping resource in the EU that addresses shipping laptops:


  Scenario 2: A package with a total of 3 or more lithium ion batteries
  contained inside of or with laptops.
  
  There is a label required on the box (see below), and the box itself
  must be of such strength as to not break or allow the batteries to
  come out or expel fumes, in other words, a good strong industrial box.





  Documentation Requirements
  
  A document is also required indicating the following
  
  
  The package contains lithium ion batteries 
  The package must be handled with care and that a flammability hazard exists if the packaged is damaged  
  If the package is damaged then special procedures including inspection and repacking may be necessary  
  A telephone number to call for additional information