how to find n-factor of h3po3

n-factor of an acid is defined as the number of H+ions replaced by 1 mole of acid in a reaction.However n-factor for acid is not equal to its basicity i.e. the number of moles of replaceable H+ atoms present in one mole of acid.
For H₃PO₃ n-factor is 1 or 2 because one of the H is not replaceable in H₃PO₃.This can be understand by observing its structure as shown below .The H atoms which are linked to oxygen are replaceable while the H atom linked directly to central atom (P) is nonreplaceable.So its n-factor is 1 or 2.