Keywords are important when it comes to internet marketing as your keywords get your website included in a search engine's results. However, there's a fine line when it comes to how many, and how often you use these words. Too many can get your site banned for spamming. The crucial part of this is to choose words and phrases that sound natural. Think of words that people searching for your goods or services might use in their search. You can then use these words in the Keyword Planner to help find other key word ideas.
You don't need to cram all the key words onto one page. That's a red-flag for search engines. All your web pages should have key words and phrases spread throughout your site's content. Remember to use these words in title tags, headings and links as well. Try not to use the same phrases, or string of words over and over. Also, change the order of the words to make the phrases unique and not robotic sounding. The idea is to optimize your search results not hinder them!
There is no magic number of times that key words need to appear on your web page. Concentrate on making sure that the words are relevant to your site, unique in the way they're presented in your text, headings and titles and that the words appear in a way that sounds natural in your content. Now go out there and make every word count!