There is no such thing as a shape count. Querying for an object requires, in fact, the traversing of whole layer (in order to skip deleted shapes, out of scope shapes, etc). But you can use:

 lastpos := layer.GetLastUid ; // first time you will call this can be time consuming

 layer.MoveFirst ;
 while ... begin
  percentage = ( layer.Shape.Uid. * 100 ) div last_pos
 end ;
 ... ]