Should you run a separate campaign for each variant?

I am watching an Amazon PPC YouTube video and the guy recommends making a separate campaign for each variant, find the winning variant, and then turn off the losers. If I run a separate campaign for each variant, won’t my variants be competing against each other though? Or is this the best practice?

Do you also run one campaign per product? For example, say you have two different gold rings. Then you should have two separate campaigns? I’m guessing that you shouldn’t separate the different size variants as their own separate campaigns, right?

This just seems like a LOT of campaigns to run, assuming you are selling a lot of different items. Thanks!