that's pretty relative- some will have almost zero, others could have a couple if it was use for demo or something like that. I don't think there is any hard and fast rule, like the miles on a car. If someone takes it for a test drive and puts 10 miles on it, it is still new.
Now, if the tractor has 8 or 10 hours, I would want to know what it had been used for, and I would probably pass it up. I would want almost no hours, just because.
No tractor is going to have zero hours.