I try to start with one high search volume keyword per marketing page (Dental Crowns) – or one highly searched full phrase per blog post (Dental Crowns vs. Veneers). From there, I use about 4-5 variations of that phrase throughout the page: Dental crowns on front teeth, Dental crown procedure, Dental crown benefits, etc.
If you find yourself putting more than this per page, it may call for the creation of a new page especially if the search volume is there (Dental Crown Cost)! I always try to keep the content direct & valuable. Trying to do too much will spread yourself thin & may overwhelm the user.
This approach has served me pretty well for about 7 years now!