Number of miles for business upgrade

You need to buy a fare that is upgrade able to begin with, which is usually 3x the average coach price in my experience.

What airline and what originating city?

It depends on the airline. That information would be helpful.

I have been somewhat successful booking a main cabin fare months in advance then watching business class fares every day to see if there’s a price drop worth moving up to business class. If there is I cancel coach and use the travel credit as at least part of the payment on business class. I haven’t done this in several years though so rules could be different now.

Thanks for info. I’ll be flying on American Airlines. Looks like American is 40-60k miles for upgrade.