If a stretched string is plucked at the centre

If the string is plucked at the mid-point, two waves move in opposite directions and get reflected at the rigid supports at the ends. The reflected waves are of same amplitude, frequency and wavelength. Superpositions of reflected waves gives rise to standing wave. At the point where the string is plucked, an antinode is always formed due to constructive super position. So, at the mid-point an antinode is always formed. This is possible if number of loops formed in the string is 1 or 3 or 5 or 7 etc.