How is the n factor of H3PO3 2???

n-factor for 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 is1 or 2 because one of the H is not replaceable in H3PO3.The H atoms which are linked to oxygen are replaceable,but H atom linked directly to central atom P is nonreplaceable.Therefore n factor for H3PO3 is 1 or 2.