In general I can fit 800 ml of water between the sight glass limits. I think the volume of the boiler is 1.6 liters, but with heat elements and space for the steam (above the sight glass), 1 liter total of water makes sense (and 1250ml to the top makes sense too).
Indeed, they say 16 cups, and to make 8 doubles (16 cups) we use around 100ml per double shot...
La Pavoni Europiccola fits around 600ml, where 400ml are usable. Hence, 8 cups (4 double shots).
Am I wrong?
Márcio.