You can work up to as many grits as you want but the most important component of mirror type paint jobs is making sure your previous grit is complete. The only way to be sure of that is to sand in alternating directions between grits. If you didn’t sand out that 400 grit scratch that’s running vertically on the your horizontal 800 grit sanding you’re not done sanding with 800. So don’t be surprised when you’ve sanded all the way with 1,000,000 grit sandpaper and see a scratch.