It depends on the distance, type of target, type of shooting (benchrest), and your actual vision. For instance, I have a 4-20x50 on my . 270 AI, I shot at a target (rock) at 966 yards on 12X and my son had to take it down to 8X. For most, 3-18/4-20 variable is plenty for a 1K yard, 10X fixed used to be the norm for snipers.
Only the best magnification for you & your situation!
For me I prefer something variable, versatile,