How high is maximum keyword density please? I've been told 6-9% is ideal. Does this change if there is more text?
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use 2-4 %

5-7% would be optimal.

I don't worry with keyword density

3-5% is ideal

Do not worry too much about this matter and try creating your web pages in a natural way.

It is always advised to develop a content for human not for search engines. Having high keyword density as like 9 % may result into high bounce rate which can harm your ranking as well. 2-5 percent is good enough

Yes, keep it at a low (2-6%) rate so it wont be keyword stuffing (spamming).

The main thing is that your keywords should be relevant to the site, describing your site, there isn't a limit of use of keywords, as long as you don't spam or stuff kws..

Try to concentrate on only 1 or 2 keywords for page. Incorporate them into you page title, meta tags, H1 tag (only 1) and the first hundred words of copy.