In Single Phase the Voltage or current will be Zero for 
every Half Cycle

In Two Phase the Votage or current in one phase is zero and 
at the same time other phase also zero and one phase get 
peak and other also get peak becuase the phase diffrence is 
180Dec. so here also no votage of current if one is zero.

In Three Phase the Phase diffrence between phases is 120Dec 
so that if one phase is zero rest of two will get some 
magnetude.So here we get Voltage or current if any one 
phase is zero.

So there is no need to generate 4phase and 5phase systems

BECAUSE THE 3 PHASE SYSTEM FOUND TO BE THE MOST ECONOMICAL 
FROM OTHER TRANSFOMATION , IN SINGLE PHASE THERE WILL 
ALWAYS BE MAGNETIC FORCES TRANSFORMER WALL WILL ALWAYS BE 
HEATED, IN 2 PHASE SYSTEM IF A WIRE BREAK HAPPENS HALF OF 
THE LOAD WILL BE DISCONECTED NOT LIKE THE 3 PHASE, AND IN 5 
OR 4 PHASE THERE WILL BE ALOT OF WIRES CONNECTION AND 
ISOLATORS USED WICH IS NOT ECONOMICAL