The forward voltage of an LED or diode is the applied voltage of the "knee" in the exponential characteristic at which it starts to substantially conduct, and in the case of an LED, produce light.
It is not, under ordinary circumstances of indicator-type LEDs, a voltage that would be produced internally to the LEDs, at least not with any substantial current capability.
As long as you are supplying the LEDs from a voltage within the Arduino's I/O range, it should not be hurt by a series strings of too many LEDs. However, in theory (if rarely in practice) a series string of too few LEDs that did not include a
current limiting resistor would stress the Arduino beyond its ratings - and possibly (and more commonly) also the LED(s) beyond their current limits.