Question

To polo composite 16 microfarad thousand volt capacitor form a supply of identical capacitors marked 8 microfarads or 20 to 50 volt the requirement of number of capacitor

Answer 1

You have capacitors rated for 250 Volts and you need the combination to have voltage rating of 1000 volts. Hence it is obvious that you should connect four numbers in series. For such a series connection, effective capacitance is  2 micro farad, because 1/Ceffective = 1/C1 + 1/C2 +1/C3 +1/C4 = 1/8 + 1/8 + 1/8 + 1/8 = 1/2.
 Therefore Ceffective  = 2 microfarad. Hence the series combination is equivalent of 2 mcrofarads, 1000 Volts. To have capacitance of 16 microfarads, you should connect 8 such series combinations in parallel, because in parallel combination, effective capacitance is equal to summation of each capacitor.  Hence capacitors required will be:
4 nos for each series combinations * 8 such combinations = 32 capacitors.