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? Is this SEO? No, it's called spamming! Don't do these activities or else Penguin will slap you.
I don't want you to fall into the same trap I fell into!
Advice: Create Facebook, Google+, Twitter, LinkedIn accounts. These should be your "social bookmarking sites". Join a community in these networks which is relevant to the site you are going to promote. Talk to the people over there, connect with them, share useful materials from the web that adds value to the group, discuss the points you learnt, ask questions, answer questions, get followers, learn what they want to see and read, write such content and share. You will see lots of discussions happening over your content, getting visits from these networks, others in the network sharing your content, resulting in "social backlinks", a good way to influence search engine rankings and ultimately resulting in getting more conversions.
Always remember any kind of "SEO" activity you are doing should help in conversions. The conversion can vary from Sales to microconversions like New visitors, a Facebook like, a Tweet, a +1, a comment on your blog post, etc. If you are not seeing any such microconversions you should double check if you are doing SEO or Spam!
