To answer the original question regarding how many times you can setback a barrel..... It depends.
Is it worth it and will you regain accuracy? Again, it depends and maybe.
Your first consideration should be do you even have enough cylinder to allow for a proper setback? Barrels with short cylinders often do not. I can't speak for other shops, but in our shop we feel if we can't at least cutoff the entire tenon for the setback, it isn't worth even trying.
Your next consideration is how much fire cracking and erosion has occurred? Again just my shop's opinion, but if there is significant erosion and the cartridge you intend to chamber might make a setback much more of a gamble and IMHO not worth doing it. There is no guaranty of performance or how much performance you'll gain from the setback. And the faster 6mm cartridges (like the 6 Creedmoor) tend to have less of a chance of regaining performance for any significant round count past the setback. Cartridges like that are just hard on barrels. We charge $175 to do a proper setback. We have done setbacks for some customers specifically on 6 Creedmoors and some guys have gotten another 500 rounds out of the barrel while other guys have gotten half that. Again, no guaranty here. It is a crap shoot.
Now, if you are thinking of going from 6CM to a more mild 6mm cartridge, you might have better results for barrel life post the setback. The other thing to consider there though is starting to work with a new cartridge on a barrel that has no guaranty on performance or life expectancy. You could spend time doing load development and getting use to the new cartridge and if performance isn't there, you don't really know how much of that could be the barrel.
All those things considered, if you are wanting to switch to a new cartridge, my best advice would be start with a new barrel. I'd burn out that 6CM barrel then make the switch.
I hope this helps you in some way.
Best regards,