From
Chapter 14 of Class XI Physics  textbook, we know that when
a string is plucked, a vast number of wavelengths are excited However only those wavelengths survive which have nodes
at the ends and form the standing wave in the string It means
that in a string, standing waves are formed when the total distance
travelled by a wave down the string and back is one wavelength, two
wavelengths, or any integral number of wavelengths Waves with other
wavelengths interfere with themselves upon reflection and their
amplitudes quickly drop to zero