You should have 50 words of text per ebay or Amazon item. The reason for this is so that your hub isn't one big spammy page of adverts (as has happened in the past).
You should really be writing a minimum of 500 words, which'd equate to 10 ebay/Amazon items, although reaching the bare minimum isn't recommended and it's good to aim for 850-1000 words.
A hub containing 1000 words of text could, therefore, have 20 ebay Modules.
"Less is more": You'd be better off having just ONE ebay module and guiding the reader through your article to the point where they really want the product you're writing about, so that when they get to that one instance they click on it and go buy it - of course, that's in the ideal world and isn't something most people could achieve in most hubs.
I hope that helps.