As many as you'd like! But generally speaking...the more, the merrier.
In order to add your own bookmarks and have them shared by other members, you must have "credits", which are earned every time you share a bookmark to each service. As you might expect, you also lose credits every time one of your own bookmarks is submitted to each social media site.
So the more social accounts you have setup, the more "credits" you'll receive, and the more submissions your own bookmarks will receive.