Tony Johnson

I have a 2004 Legacy Sedan, 11700 miles. I recently acquired a nail in one tire. Haven't determined if the tire is repairable but in case it is not I want to learn what the tire replacement options I have. I understand uneven relative thread wear between tires can be detrimental to the drive train and braking system. What are the tech specs regarding replacement of tires on a 2004 Subaru Legacy sedan? Is there a thread wear differential spec that would allow replacing less than all the tires, say only 2 tires or perhaps 1 tire? Tony Johnson