Think 10 will do! or 100 or you can even go for a 1000! But think about how much traffic you will get from such sites? Probably zero. How much influence it will have on your SEO campaign? Zero, remember Penguin. It's not about how many social bookmarks. It's about how many visits and conversions you are going to get from such bookmarks. Full disclosure: At the early years of my career I used to do bookmarkings in 100s of websites. I thought I was doing SEO. Each day I will pick 2 links from the website and go to these 100s of bookmarking sites and share it. But one fine day I decided to check analytics to find how much visitors I get from such activities. I realized it was zero. Yes, zero! Okay, maybe it was influencing search engine rankings. Oh, really? So the answer is maybe it was influencing the rankings, but Google clearly says it's a kind of link spamming to trick the algorithm. Once Google catches it, you will be lost in the SERP. So what was I doing?...