How many plays make up the triology of dramas designated as Wallenstein by German author Friedrich Schiller?
return German author ;return #1 that is Friedrich Schiller ;return triology of dramas of  #2 ;return #3 that are designated as Wallenstein ;return plays in  #4 ;return number of  #5
['select', 'filter', 'project', 'filter', 'project', 'aggregate']