For articles: I would take about 1-15% of keyword density

No more than 24 (4%). If it's a long keyphrase, even fewer. If the article is for EZA, keyword density cannot be over 2% so if u plan to submit there, no more than 8-12 times.

You can make it 2-8%. And if you want we can provide you SEO optimized articles.

1 at the beginning of article, 2 in the middle and 1 at the end of the content. Include about 3-5 related / LSI keywords within content to form a keyword cluster basing around your primary keyword.

I hope there is no definite keyword density defined for Google and other search engines. A few weeks before I have searched about the same info and unable to find the right keyword desnsity. You should reduce the keyword density BEFORE your article looks spammy.

Well according to my experience it should not be more than 2-3% so you can use your main keyword in your whole content for at least 12-18 times.

keyword density should be about 4%, so 24 times for 600 word article is enough.

Keyword density is largely irrelevant nowadays, as it's very easy to fake - G started disregarding it a long time ago.

I usually only 2% -3%

keyword density should be 2 to 3%

I like to keep mine at less than 2 percent that way you know ur not pissing any SE off
but in comparison to backlinks I think yes it is irrelevant as a ranking factor