How many, which sites?
Implementation
I am a highly experience link builder. The hows whys and whats of link building is a simple science that can be explained in a few minutes, however the implementation is a slow time consuming exercise that is forever at the mercy of Google's algorithm and other webmasters.
Exactly how many links do you need?
A simple test is type into Google a keyword or phrase that best describes your business. Copy the url of the website that ranks #1.. Return to Google and type in link:www.domain.co.uk. substituting domain.co.uk with the top ranked website's web address. Record the number of linking pages.
Although it is quality of website (often dictated by Page rank) not the quantity of links that are the deciding factor, what makes a "high quality" website is the questionable factor and up to interpretation.