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Having many links on a page will not reduce the PageRank of the page that it's on. However, it will reduce the amount of PageRank being passed to the individual links on the page, so you should try to keep the number as low as you can while maintaining a good user experience. A general rule of thumb is to keep the number of links to 100 or less, although many ecommerce sites have many more (the typical Amazon.com page has 400+ links). Linking to high-quality, relevant external sites is believed to improve the relevancy of your own page. These should not be nofollowed. In general, it should not be necessary to use nofollow for external links unless you are linking to a direct competitor (but then, why would you be linking to them?) or in the case of UGC where people may leave links to low quality sites. In general, just use your common sense and link to any relevant internal or external pages that you think will enhance the value of your content. Don't use nofollow. It's good to use target="_blank" to have external links open in a new window.