Maximum voltage that i can input in an i/o

Which is the max voltage i can input into an i/o pin

For which Arduino board ?
Can you tell what your project is ?

In theory it is 5.5V for an Arduino Uno, but it is a bad idea to connect a voltage to a Arduino pin (without limiting the current). Often a protection resistor is used.
A common mistake is when the voltage of a Lipo cell is connected to an analog input pin and the Arduino is turned off. That can damage the Arduino board.

Assuming Arduino UNO or Pro Mini (but this probably applies for all):

From the ATmega328P datasheet:

VIH Input High Voltage, except XTAL1 and RESET pins: VCC + 0.5 V

And Koepal makes some good points, too. Didn't know about the "battery on input at power down" thing, but makes sense.