In order to achieve targeted search engine rankings, you will need to obtain a number of links into your site.

How many links you need to obtain?
Howe many links you will have to get depends on how many links go to your competitors. First, go to the search engine you are going to optimize for, and search the links your competitors are getting. For example, if your competitor's website is http://www.competitor1.com and you are marketing on Yahoo, input the URL into the search box as link:http://www.competitor1.com, then it shows the number of links leading to your competitor.

If you want to rank higher than your competitor, it is safe to get at least a few more links than your competitors. And so does for Google and MSN search engines if you want to obtain high rankings on them.

Not every link counts the same.
Links from different sites or web pages are different in value for your link swap campaign. In general, the more links to that page, the more value of link from that page. The more pages indexed, the more value of links from that site.