The number of SEO keywords you should use depends on your goals and resources available. But the key is not really the number of keywords used but how they are used. Keywords should be included based on each individual piece of content on a site rather than throughout a website as a whole. Search engines associate this approach with higher-quality content and therefore consider it more valuable when ranking.
Additionally, each piece of content should be unique with differentiated keywords. Publishing the same information with only slight variations and the same keywords won’t have much of an effect on rankings. Keywords need to be highly specific to the content being created. For example, just having “SEO” as a keyword in a piece of content dooms it to drown in an ocean of other articles about SEO. Be precise with your keywords, and search for them on Google beforehand to find combinations that will not produce an overwhelming amount of competition.