How do they ensure there is one electron on an oil drop in Millikan's oil drop experiment?
There does not have to be just one electron per drop.  Say you have a drop which, in reality, picked up four electrons, another with five and a third drop with seven.  None has one electronic charge, but when you measure charges you find they have a common factor; the first drop has four times that factor, the second has five times that factor and the third drop shows a multiplier of seven.  It was this common factor, not necessarily the charge on any specific drop, that was recognized as one electronic charge.