How many posts does it take on a specific keyword to start ranking for it? On average anyway.
Yes you must do that and for that you can use blog search also . Like you have created blog post on photoshop brushes than go to blog search and search for photoshop brushes .
It will display all latest posts published world wide on your keyword . Now open each post related to your content and start posting good replies . Use your keyword as your site name .
The best sources for blog link building are
1) Dofollow social bookmarks
2) Rss directories
3) Dofollow blog comments
5) Blog directories
With quality content, you should get some backlinks from trackbacks and sites picking up your feed.
Write an article ( 300-400 words ) and submit in 20 high PR directories to get 40 one way backlinks.
Here is verified Quality Article Directories list.
Google Directory - Business*>*Publishing*and*Printing*>*Publishing*>* Services*>*Free*Content
Check this thread for RSS directories.