Presuming that this tree is connected into a mains power source in the UK. The total output of the UK mains source is 230V. So if we share this voltage equally between 50 bulbs:

I'm not 100 per cent sure this is correct though and different countries may have different mains power supplies. Hope it helps anyway.
EDIT: This also does not take into account resistance in the bulbs and wires but is all I can tell you from the information provided in the question.