You can place an amazon and an ebay ad per 250 words in your hub. The products you place must be related to the content of your hub. Keep in mind that placing to much of ads will make your hub over promotional and it may be automatically unpublished. Also too much of ads would make your hubs appear unpleasent. So it will be better to place maximum of 3 amazon and 3 ebay ads in each hub so that it would have a decent look.