Looking for tips/advice!

I'm thinking about opening an Amazon store. Just wondering if anyone's familiar with Amazon marketplace and has any tips!

With the individual seller package, can you only have 20 items posted in your store at a time? Or does it literally just mean you can't sell any more then 20 items in a month? Jw because I'm thinking about starting out with that plan just to see how it does. But I would like to have more than 20 items available.

I was also wondering what the additional sellers fees are like? Can you actually make money or do they end up taking a big percentage?

TIA!!!😊