If you join Google Adsense, then Adsense puts the ads on your hubs for you. If you join Amazon.com; then you have to put an amazon capsule into your hub, and you have to pick what sort of products that you want to put for sale in that capsule, and you can then place it anywhere you want on your hub. You can probably have as many amazon.com capsules as you want in a hub. I doubt that there is a limit, of course too many wouldn't look so nice.
Look at the help section at the bottom of the page to learn how to do these things. Hubpages has every single thing explained there. When I started I didn't bother to read any of that stuff either though.