You do not have an N number of keywords that are considered ideal to advertise on Google AdWords. The question that you should ask yourself is: How specific do I want my targeting strategy to be?
Generally, the more specific your advertising strategy is the better results you will get.
Let me clarify this point with a specific example regarding an insurance businesses in Australia.
An insurance business can have the interest of getting different types of clients:
==> Commercial Insurance
==> Car Insurance
==> Moto Insurance
==> Home Insurance
==> Life insurance
==> Renter insurance
You can approach a potential AdWords account in 2 ways:
For instance, an efficient campaign structure could be:
Campaign #1: Home Insurance
Home Insurance: All Keywords containing the terms home insurance in it
House Insurance: All Keywords containing the terms house insurance in it
Cheap Home Insurance: All keywords containing the terms cheap home insurance in it
Cheap House Insurance: All keywords containing the terms cheap house insurance in it.
Compare House Insurance: All keywords talking about comparing house insurance
Compare Home Insurance: All keywords talking about comparing home insurance
The method we just described requires a bigger effort but will pay off way more than a less specific account structure.