One thing that I forgot to point out is it depends on what product you are using because some have a mild cleaner and while applying a 2 coat you will remove part of the first coat.

I talked to the chemist at Duragloss and he said after 2 coats you will Begin to mute the clarity.
Whatever you use 2 coats are adequate but many do like to apply multi-coats of wax but the gains are more in making them feel like they are doing something more than actually doing anything to improve the protection.
You ask about a sealant but it should be noted that if you use a wax then you will need to redo it much more often due to short life of wax.