How many antidepressants would a person need to take to die?

Surprisingly few. As antidepressants are given to treat depression, medications that are particularly lethal in smaller doses present obvious problems. As such, I am not aware of an antidepressant that has a lethal dosage that isn't huge, and unobtainable.

Do note however, that overdosing on anything may have unforeseen and uncomfortable effects, other than lethality.