monophobia (n.)

word-forming element of Greek origin meaning "one, single, alone; containing one (atom, etc.)," from Greek monos "single, alone," from PIE root *men- (4) "small, isolated."

word-forming element meaning "excessive or irrational fear, horror, or aversion," from Latin -phobia and directly from Greek -phobia "panic fear of," from phobos "fear" (see phobia). In widespread popular use with native words from c. 1800. In psychology, "an abnormal or irrational fear." Related: -phobic.