Need help designing headphone amplifier circuit
Voltage in x gain = voltage out. The current that flows in the headphones is V/Z where Z is impedance of headphones. Z is a little frequency dependent.

Input current to amplifier is voltage in/ input impedance. Input impedance is usually 1k to 1Mohm (generalism warning).

Yes, you need to know the input voltage. You need to know the maximum it can attain and the nominal RMS value that would produce (say) a comfortable nominal listening level in the headphones. The headphones will have a SPL (sound pressure level) characteristic that you need to research. Also research what would be a comfortable SPL level. You then begin to know what the voltage level ought to be across each speaker in the headphones.

You might also consider hard-clipping to prevent acoustic shock (should your amplifier be capable of delivering much higher voltages to the head phones).