There isn’t an optimum number. We can optimise for as many keyword search terms as you like over the course of the project as some projects can run for years. However the optimum number of keywords for your website is dependent on the level of competition, the results of visits once your site ranks well (enquiries + conversions) as well as the available budget.
It depends on the goals of your website, some just want to be found and read, a blog for example, whilst if selling products, the aim is going to be sales and which keywords are going to convert best.
Some SEO’s will advise using what is called long tail keywords and lot’s of them, whilst others will advise using more competitive short tail keywords with a more stable monthly search volume.
> For Example :
– Long tail keyword : Blank CD supplier in Central London
– Short tail keyword : Blank CD
The best way is to use a combination of both and re adjust accordingly when the need arises as there are potential problems with both.
Long tail keywords might not have sufficient demand and visits might not lead to enquiries or conversions. Short tail keywords can bring lots of visits but can have different connation’s so might bring the wrong type of visitors to your site.
It is now a lot more important to choose the correct keywords initially, than it was a few years ago, due to changes to the criteria Google and Bing use to rank webpages.
Keywords can be tested using PPC such as Google Adwords initially, with those that do well, being used for SEO.
A good SEO should advise using a combination of both for best results for your website, be flexible and change the focus of your project if the need arises when feasible.