Search Engine Optimization requires a systematic approach in which estimating the number of links for each keyword is not an easy task. Making a wrong estimate can burn big hole in your pocket and will not achieve the expected results in terms of Search Engine Placements as well.
There are various factors which one should consider to estimate this number.
1. The competition for the keyword.
2. Links pointing to the websites in Top 5 for Search Results for keyword.
3. Estimated Traffic numbers for keyword.
4. Quality of links you are planning to achieve.
I will now explain all this using an example.
Let us choose a keyword "wedding favors"
If you make a Search on Google for this keyword, you will get the total number of results as 5,530,000.
At the time I am writing this article, the search results show http://www.favorsbyserendipity.com/ as # 1 result.
For this website, there are 323 pages linking to their website as reported by Google, This can be found by making a search for word link:http://www.favorsbyserendipity.com/ on Google.
You can do a similar check for other websites that come in top 5 or top 10 results to get a firm idea.
To get estimated traffic for these keywords you can use a nice tool provided by digitalpoint. You can find it here http://www.digitalpoint.com/tools/suggestion
Links are of two main types
a. one way links (considered more important these days)
b. reciprocal links (considered less important these days)
Links Quality depends on various factors like relevance, page rank, number of outgoing links and frequency of caching of the web page on which your link is place. This will be discussed in details in next article.
I hope a new webmaster will get an overview about calculation on the number of links required for a specific keyword for Top Placements on Google and Yahoo.