To estimate the state of the knowledge base as a whole, you'll need a representative amount of articles for evaluation.
If your team creates numerous articles, you may use statistical calculators (e.g.
this one) to find a proper sample size.
Otherwise, one may simply consider 10-15% a reasonable starting point.
You may ask: 10-15% of what? Good question!
The base is the number of public articles. Only public articles, not drafts, WIPs, etc.
The reason is that your knowledge base should be ranked based on criteria related to your customers and their experiences.
Quality standards for internal articles would typically be much lower, so there is no need to include those in the quality evaluation process.
Since we are talking about KCS methodology, it suggests that articles are to be created, published, and modified as necessary. So the article should be added for evaluation if either of these two events occurs: publication or modification.